[v4,1/1] oslib-posix: initialize backend memory objects in parallel

QEMU initializes preallocated backend memory as the objects are parsed from
the command line. This is not optimal in some cases (e.g. memory spanning
multiple NUMA nodes) because the memory objects are initialized in series.

Allow the initialization to occur in parallel (asynchronously). In order to
ensure optimal thread placement, asynchronous initialization requires prealloc
context threads to be in use.

Signed-off-by: Mark Kanda <mark.kanda@oracle.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 backends/hostmem.c     |   7 ++-
 hw/virtio/virtio-mem.c |   4 +-
 include/hw/qdev-core.h |   5 ++
 include/qemu/osdep.h   |  18 +++++-
 system/vl.c            |   9 +++
 util/oslib-posix.c     | 131 +++++++++++++++++++++++++++++++----------
 util/oslib-win32.c     |   8 ++-
 7 files changed, 145 insertions(+), 37 deletions(-)

On 31.01.24 17:53, Mark Kanda wrote:
> QEMU initializes preallocated backend memory as the objects are parsed from
> the command line. This is not optimal in some cases (e.g. memory spanning
> multiple NUMA nodes) because the memory objects are initialized in series.
> 
> Allow the initialization to occur in parallel (asynchronously). In order to
> ensure optimal thread placement, asynchronous initialization requires prealloc
> context threads to be in use.
> 
> Signed-off-by: Mark Kanda <mark.kanda@oracle.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---

So, this LGTM. There might be ways to not rely on phases to achieve what 
we want to achieve (e.g., let the machine set an internal property on 
memory backends we create from the cmdline), but this should do as well.

I'll wait a bit for more feedback. If there is none, I'll route this 
through my tree (after doing a quick sanity test).

Thanks!

This patch has been successfully tested by QE. After configuring two
memory-backends with preallocation context objects, binded to two host
nodes; the result is QEMU being at least three times faster than
before.

# time /usr/libexec/qemu-kvm -M q35 -m 16G,maxmem=32G -numa
node,memdev=mem0,nodeid=0 -numa node,memdev=mem1,nodeid=1 -object
thread-context,id=tc1,node-affinity=0 -object
thread-context,id=tc2,node-affinity=1 -object
memory-backend-ram,id=mem0,size=8G,policy=bind,host-nodes=0 -object
memory-backend-ram,id=mem1,size=8G,policy=bind,host-nodes=1 -nographic
-monitor stdio
QEMU 8.2.0 monitor - type 'help' for more information
qemu-kvm: cannot use stdio by multiple character devices
qemu-kvm: could not connect serial device to character backend 'stdio'

real 0m0.038s
user 0m0.013s
sys 0m0.005s

# time /home/qemu/build/qemu-system-x86_64 -M q35 -m 16G,maxmem=32G
-numa node,memdev=mem0,nodeid=0 -numa node,memdev=mem1,nodeid=1
-object thread-context,id=tc1,node-affinity=0 -object
thread-context,id=tc2,node-affinity=1 -object
memory-backend-ram,id=mem0,size=8G,policy=bind,host-nodes=0 -object
memory-backend-ram,id=mem1,size=8G,policy=bind,host-nodes=1 -nographic
-monitor stdio
QEMU 8.2.50 monitor - type 'help' for more information
qemu-system-x86_64: cannot use stdio by multiple character devices
qemu-system-x86_64: could not connect serial device to character backend 'stdio'

real 0m0.012s
user 0m0.006s
sys 0m0.007s

Tested-by: Mario Casquero <mcasquer@redhat.com>


On Wed, Jan 31, 2024 at 7:24 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 31.01.24 17:53, Mark Kanda wrote:
> > QEMU initializes preallocated backend memory as the objects are parsed from
> > the command line. This is not optimal in some cases (e.g. memory spanning
> > multiple NUMA nodes) because the memory objects are initialized in series.
> >
> > Allow the initialization to occur in parallel (asynchronously). In order to
> > ensure optimal thread placement, asynchronous initialization requires prealloc
> > context threads to be in use.
> >
> > Signed-off-by: Mark Kanda <mark.kanda@oracle.com>
> > Signed-off-by: David Hildenbrand <david@redhat.com>
> > ---
>
> So, this LGTM. There might be ways to not rely on phases to achieve what
> we want to achieve (e.g., let the machine set an internal property on
> memory backends we create from the cmdline), but this should do as well.
>
> I'll wait a bit for more feedback. If there is none, I'll route this
> through my tree (after doing a quick sanity test).
>
> Thanks!
>
> --
> Cheers,
>
> David / dhildenb
>
>

On 1/31/24 08:53, Mark Kanda wrote:
> QEMU initializes preallocated backend memory as the objects are parsed from
> the command line. This is not optimal in some cases (e.g. memory spanning
> multiple NUMA nodes) because the memory objects are initialized in series.
> 
> Allow the initialization to occur in parallel (asynchronously). In order to
> ensure optimal thread placement, asynchronous initialization requires prealloc
> context threads to be in use.
> 
> Signed-off-by: Mark Kanda <mark.kanda@oracle.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  backends/hostmem.c     |   7 ++-
>  hw/virtio/virtio-mem.c |   4 +-
>  include/hw/qdev-core.h |   5 ++
>  include/qemu/osdep.h   |  18 +++++-
>  system/vl.c            |   9 +++
>  util/oslib-posix.c     | 131 +++++++++++++++++++++++++++++++----------
>  util/oslib-win32.c     |   8 ++-
>  7 files changed, 145 insertions(+), 37 deletions(-)
> 
> diff --git a/backends/hostmem.c b/backends/hostmem.c
> index 30f69b2cb5..17221e422a 100644
> --- a/backends/hostmem.c
> +++ b/backends/hostmem.c
> @@ -20,6 +20,7 @@
>  #include "qom/object_interfaces.h"
>  #include "qemu/mmap-alloc.h"
>  #include "qemu/madvise.h"
> +#include "hw/qdev-core.h"
>  
>  #ifdef CONFIG_NUMA
>  #include <numaif.h>
> @@ -237,7 +238,7 @@ static void host_memory_backend_set_prealloc(Object *obj, bool value,
>          uint64_t sz = memory_region_size(&backend->mr);
>  
>          if (!qemu_prealloc_mem(fd, ptr, sz, backend->prealloc_threads,
> -                               backend->prealloc_context, errp)) {
> +                               backend->prealloc_context, false, errp)) {
>              return;
>          }
>          backend->prealloc = true;
> @@ -323,6 +324,7 @@ host_memory_backend_memory_complete(UserCreatable *uc, Error **errp)
>      HostMemoryBackendClass *bc = MEMORY_BACKEND_GET_CLASS(uc);
>      void *ptr;
>      uint64_t sz;
> +    bool async = !phase_check(PHASE_LATE_BACKENDS_CREATED);
>  
>      if (!bc->alloc) {
>          return;
> @@ -398,7 +400,8 @@ host_memory_backend_memory_complete(UserCreatable *uc, Error **errp)
>      if (backend->prealloc && !qemu_prealloc_mem(memory_region_get_fd(&backend->mr),
>                                                  ptr, sz,
>                                                  backend->prealloc_threads,
> -                                                backend->prealloc_context, errp)) {
> +                                                backend->prealloc_context,
> +                                                async, errp)) {
>          return;
>      }
>  }
> diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
> index 99ab989852..ffd119ebac 100644
> --- a/hw/virtio/virtio-mem.c
> +++ b/hw/virtio/virtio-mem.c
> @@ -605,7 +605,7 @@ static int virtio_mem_set_block_state(VirtIOMEM *vmem, uint64_t start_gpa,
>          int fd = memory_region_get_fd(&vmem->memdev->mr);
>          Error *local_err = NULL;
>  
> -        if (!qemu_prealloc_mem(fd, area, size, 1, NULL, &local_err)) {
> +        if (!qemu_prealloc_mem(fd, area, size, 1, NULL, false, &local_err)) {
>              static bool warned;
>  
>              /*
> @@ -1248,7 +1248,7 @@ static int virtio_mem_prealloc_range_cb(VirtIOMEM *vmem, void *arg,
>      int fd = memory_region_get_fd(&vmem->memdev->mr);
>      Error *local_err = NULL;
>  
> -    if (!qemu_prealloc_mem(fd, area, size, 1, NULL, &local_err)) {
> +    if (!qemu_prealloc_mem(fd, area, size, 1, NULL, false, &local_err)) {
>          error_report_err(local_err);
>          return -ENOMEM;
>      }
> diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
> index 151d968238..83dd9e2485 100644
> --- a/include/hw/qdev-core.h
> +++ b/include/hw/qdev-core.h
> @@ -1071,6 +1071,11 @@ typedef enum MachineInitPhase {
>       */
>      PHASE_ACCEL_CREATED,
>  
> +    /*
> +     * Late backend objects have been created and initialized.
> +     */
> +    PHASE_LATE_BACKENDS_CREATED,
> +
>      /*
>       * machine_class->init has been called, thus creating any embedded
>       * devices and validating machine properties.  Devices created at
> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> index c9692cc314..7d359dabc4 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -680,6 +680,8 @@ typedef struct ThreadContext ThreadContext;
>   * @area: start address of the are to preallocate
>   * @sz: the size of the area to preallocate
>   * @max_threads: maximum number of threads to use
> + * @tc: prealloc context threads pointer, NULL if not in use
> + * @async: request asynchronous preallocation, requires @tc
>   * @errp: returns an error if this function fails
>   *
>   * Preallocate memory (populate/prefault page tables writable) for the virtual
> @@ -687,10 +689,24 @@ typedef struct ThreadContext ThreadContext;
>   * each page in the area was faulted in writable at least once, for example,
>   * after allocating file blocks for mapped files.
>   *
> + * When setting @async, allocation might be performed asynchronously.
> + * qemu_finish_async_prealloc_mem() must be called to finish any asynchronous
> + * preallocation.
> + *
>   * Return: true on success, else false setting @errp with error.
>   */
>  bool qemu_prealloc_mem(int fd, char *area, size_t sz, int max_threads,
> -                       ThreadContext *tc, Error **errp);
> +                       ThreadContext *tc, bool async, Error **errp);
> +
> +/**
> + * qemu_finish_async_prealloc_mem:
> + * @errp: returns an error if this function fails
> + *
> + * Finish all outstanding asynchronous memory preallocation.
> + *
> + * Return: true on success, else false setting @errp with error.
> + */
> +bool qemu_finish_async_prealloc_mem(Error **errp);
>  
>  /**
>   * qemu_get_pid_name:
> diff --git a/system/vl.c b/system/vl.c
> index 788d88ea03..e6bc5d9dd9 100644
> --- a/system/vl.c
> +++ b/system/vl.c
> @@ -2009,6 +2009,14 @@ static void qemu_create_late_backends(void)
>  
>      object_option_foreach_add(object_create_late);
>  
> +    /*
> +     * Wait for any outstanding memory prealloc from created memory
> +     * backends to complete.
> +     */
> +    if (!qemu_finish_async_prealloc_mem(&error_fatal)) {
> +        exit(1);
> +    }
> +
>      if (tpm_init() < 0) {
>          exit(1);
>      }
> @@ -3695,6 +3703,7 @@ void qemu_init(int argc, char **argv)
>       * over memory-backend-file objects).
>       */
>      qemu_create_late_backends();
> +    phase_advance(PHASE_LATE_BACKENDS_CREATED);
>  
>      /*
>       * Note: creates a QOM object, must run only after global and
> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
> index 7c297003b9..dada4722f6 100644
> --- a/util/oslib-posix.c
> +++ b/util/oslib-posix.c
> @@ -42,6 +42,7 @@
>  #include "qemu/cutils.h"
>  #include "qemu/units.h"
>  #include "qemu/thread-context.h"
> +#include "qemu/main-loop.h"
>  
>  #ifdef CONFIG_LINUX
>  #include <sys/syscall.h>
> @@ -63,11 +64,15 @@
>  
>  struct MemsetThread;
>  
> +static QLIST_HEAD(, MemsetContext) memset_contexts =
> +    QLIST_HEAD_INITIALIZER(memset_contexts);
> +
>  typedef struct MemsetContext {
>      bool all_threads_created;
>      bool any_thread_failed;
>      struct MemsetThread *threads;
>      int num_threads;
> +    QLIST_ENTRY(MemsetContext) next;
>  } MemsetContext;
>  
>  struct MemsetThread {
> @@ -412,19 +417,44 @@ static inline int get_memset_num_threads(size_t hpagesize, size_t numpages,
>      return ret;
>  }
>  
> +static int wait_and_free_mem_prealloc_context(MemsetContext *context)
> +{
> +    int i, ret = 0, tmp;
> +
> +    for (i = 0; i < context->num_threads; i++) {
> +        tmp = (uintptr_t)qemu_thread_join(&context->threads[i].pgthread);
> +
> +        if (tmp) {
> +            ret = tmp;
> +        }
> +    }
> +    g_free(context->threads);
> +    g_free(context);
> +    return ret;
> +}
> +
>  static int touch_all_pages(char *area, size_t hpagesize, size_t numpages,
> -                           int max_threads, ThreadContext *tc,
> +                           int max_threads, ThreadContext *tc, bool async,
>                             bool use_madv_populate_write)
>  {
>      static gsize initialized = 0;
> -    MemsetContext context = {
> -        .num_threads = get_memset_num_threads(hpagesize, numpages, max_threads),
> -    };
> +    MemsetContext *context = g_malloc0(sizeof(MemsetContext));
>      size_t numpages_per_thread, leftover;
>      void *(*touch_fn)(void *);
> -    int ret = 0, i = 0;
> +    int ret, i = 0;
>      char *addr = area;
>  
> +    /*
> +     * Asynchronous preallocation is only allowed when using MADV_POPULATE_WRITE
> +     * and prealloc context for thread placement.
> +     */
> +    if (!use_madv_populate_write || !tc) {
> +        async = false;
> +    }
> +
> +    context->num_threads =
> +        get_memset_num_threads(hpagesize, numpages, max_threads);
> +
>      if (g_once_init_enter(&initialized)) {
>          qemu_mutex_init(&page_mutex);
>          qemu_cond_init(&page_cond);
> @@ -432,8 +462,11 @@ static int touch_all_pages(char *area, size_t hpagesize, size_t numpages,
>      }
>  
>      if (use_madv_populate_write) {
> -        /* Avoid creating a single thread for MADV_POPULATE_WRITE */
> -        if (context.num_threads == 1) {
> +        /*
> +         * Avoid creating a single thread for MADV_POPULATE_WRITE when
> +         * preallocating synchronously.
> +         */
> +        if (context->num_threads == 1 && !async) {
>              if (qemu_madvise(area, hpagesize * numpages,
>                               QEMU_MADV_POPULATE_WRITE)) {
>                  return -errno;
> @@ -445,50 +478,86 @@ static int touch_all_pages(char *area, size_t hpagesize, size_t numpages,
>          touch_fn = do_touch_pages;
>      }
>  
> -    context.threads = g_new0(MemsetThread, context.num_threads);
> -    numpages_per_thread = numpages / context.num_threads;
> -    leftover = numpages % context.num_threads;
> -    for (i = 0; i < context.num_threads; i++) {
> -        context.threads[i].addr = addr;
> -        context.threads[i].numpages = numpages_per_thread + (i < leftover);
> -        context.threads[i].hpagesize = hpagesize;
> -        context.threads[i].context = &context;
> +    context->threads = g_new0(MemsetThread, context->num_threads);
> +    numpages_per_thread = numpages / context->num_threads;
> +    leftover = numpages % context->num_threads;
> +    for (i = 0; i < context->num_threads; i++) {
> +        context->threads[i].addr = addr;
> +        context->threads[i].numpages = numpages_per_thread + (i < leftover);
> +        context->threads[i].hpagesize = hpagesize;
> +        context->threads[i].context = context;
>          if (tc) {
> -            thread_context_create_thread(tc, &context.threads[i].pgthread,
> +            thread_context_create_thread(tc, &context->threads[i].pgthread,
>                                           "touch_pages",
> -                                         touch_fn, &context.threads[i],
> +                                         touch_fn, &context->threads[i],
>                                           QEMU_THREAD_JOINABLE);
>          } else {
> -            qemu_thread_create(&context.threads[i].pgthread, "touch_pages",
> -                               touch_fn, &context.threads[i],
> +            qemu_thread_create(&context->threads[i].pgthread, "touch_pages",
> +                               touch_fn, &context->threads[i],
>                                 QEMU_THREAD_JOINABLE);
>          }
> -        addr += context.threads[i].numpages * hpagesize;
> +        addr += context->threads[i].numpages * hpagesize;
> +    }
> +
> +    if (async) {
> +        /*
> +         * async requests currently require the BQL. Add it to the list and kick
> +         * preallocation off during qemu_finish_async_prealloc_mem().
> +         */
> +        assert(bql_locked());
> +        QLIST_INSERT_HEAD(&memset_contexts, context, next);
> +        return 0;
>      }
>  
>      if (!use_madv_populate_write) {
> -        sigbus_memset_context = &context;
> +        sigbus_memset_context = context;
>      }
>  
>      qemu_mutex_lock(&page_mutex);
> -    context.all_threads_created = true;
> +    context->all_threads_created = true;
>      qemu_cond_broadcast(&page_cond);
>      qemu_mutex_unlock(&page_mutex);
>  
> -    for (i = 0; i < context.num_threads; i++) {
> -        int tmp = (uintptr_t)qemu_thread_join(&context.threads[i].pgthread);
> +    ret = wait_and_free_mem_prealloc_context(context);
>  
> +    if (!use_madv_populate_write) {
> +        sigbus_memset_context = NULL;
> +    }
> +    return ret;
> +}
> +
> +bool qemu_finish_async_prealloc_mem(Error **errp)
> +{
> +    int ret, tmp;

The above should be initialized?

I did a build test and encounter:

In file included from ../util/oslib-posix.c:36:
../util/oslib-posix.c: In function ‘qemu_finish_async_prealloc_mem’:
/home/libvirt/vm/software/qemu/include/qapi/error.h:334:5: error: ‘ret’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  334 |     error_setg_errno_internal((errp), __FILE__, __LINE__, __func__,     \
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~
../util/oslib-posix.c:531:9: note: ‘ret’ was declared here
  531 |     int ret, tmp;
      |         ^~~
cc1: all warnings being treated as errors
ninja: build stopped: subcommand failed.
make: *** [Makefile:162: run-ninja] Error 1

Thank you very much!

Dongli Zhang

> +    MemsetContext *context, *next_context;
> +
> +    /* Waiting for preallocation requires the BQL. */
> +    assert(bql_locked());
> +    if (QLIST_EMPTY(&memset_contexts)) {
> +        return true;
> +    }
> +
> +    qemu_mutex_lock(&page_mutex);
> +    QLIST_FOREACH(context, &memset_contexts, next) {
> +        context->all_threads_created = true;
> +    }
> +    qemu_cond_broadcast(&page_cond);
> +    qemu_mutex_unlock(&page_mutex);
> +
> +    QLIST_FOREACH_SAFE(context, &memset_contexts, next, next_context) {
> +        QLIST_REMOVE(context, next);
> +        tmp = wait_and_free_mem_prealloc_context(context);
>          if (tmp) {
>              ret = tmp;
>          }
>      }
>  
> -    if (!use_madv_populate_write) {
> -        sigbus_memset_context = NULL;
> +    if (ret) {
> +        error_setg_errno(errp, -ret,
> +                         "qemu_prealloc_mem: preallocating memory failed");
> +        return false;
>      }
> -    g_free(context.threads);
> -
> -    return ret;
> +    return true;
>  }
>  
>  static bool madv_populate_write_possible(char *area, size_t pagesize)
> @@ -498,7 +567,7 @@ static bool madv_populate_write_possible(char *area, size_t pagesize)
>  }
>  
>  bool qemu_prealloc_mem(int fd, char *area, size_t sz, int max_threads,
> -                       ThreadContext *tc, Error **errp)
> +                       ThreadContext *tc, bool async, Error **errp)
>  {
>      static gsize initialized;
>      int ret;
> @@ -540,7 +609,7 @@ bool qemu_prealloc_mem(int fd, char *area, size_t sz, int max_threads,
>      }
>  
>      /* touch pages simultaneously */
> -    ret = touch_all_pages(area, hpagesize, numpages, max_threads, tc,
> +    ret = touch_all_pages(area, hpagesize, numpages, max_threads, tc, async,
>                            use_madv_populate_write);
>      if (ret) {
>          error_setg_errno(errp, -ret,
> diff --git a/util/oslib-win32.c b/util/oslib-win32.c
> index c4a5f05a49..b623830d62 100644
> --- a/util/oslib-win32.c
> +++ b/util/oslib-win32.c
> @@ -265,7 +265,7 @@ int getpagesize(void)
>  }
>  
>  bool qemu_prealloc_mem(int fd, char *area, size_t sz, int max_threads,
> -                       ThreadContext *tc, Error **errp)
> +                       ThreadContext *tc, bool async, Error **errp)
>  {
>      int i;
>      size_t pagesize = qemu_real_host_page_size();
> @@ -278,6 +278,12 @@ bool qemu_prealloc_mem(int fd, char *area, size_t sz, int max_threads,
>      return true;
>  }
>  
> +bool qemu_finish_async_prealloc_mem(Error **errp)
> +{
> +    /* async prealloc not supported, there is nothing to finish */
> +    return true;
> +}
> +
>  char *qemu_get_pid_name(pid_t pid)
>  {
>      /* XXX Implement me */

On 03.02.24 23:43, Dongli Zhang wrote:
> 
> 
> On 1/31/24 08:53, Mark Kanda wrote:
>> QEMU initializes preallocated backend memory as the objects are parsed from
>> the command line. This is not optimal in some cases (e.g. memory spanning
>> multiple NUMA nodes) because the memory objects are initialized in series.
>>
>> Allow the initialization to occur in parallel (asynchronously). In order to
>> ensure optimal thread placement, asynchronous initialization requires prealloc
>> context threads to be in use.
>>
>> Signed-off-by: Mark Kanda <mark.kanda@oracle.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>   backends/hostmem.c     |   7 ++-
>>   hw/virtio/virtio-mem.c |   4 +-
>>   include/hw/qdev-core.h |   5 ++
>>   include/qemu/osdep.h   |  18 +++++-
>>   system/vl.c            |   9 +++
>>   util/oslib-posix.c     | 131 +++++++++++++++++++++++++++++++----------
>>   util/oslib-win32.c     |   8 ++-
>>   7 files changed, 145 insertions(+), 37 deletions(-)
>>
>> diff --git a/backends/hostmem.c b/backends/hostmem.c
>> index 30f69b2cb5..17221e422a 100644
>> --- a/backends/hostmem.c
>> +++ b/backends/hostmem.c
>> @@ -20,6 +20,7 @@
>>   #include "qom/object_interfaces.h"
>>   #include "qemu/mmap-alloc.h"
>>   #include "qemu/madvise.h"
>> +#include "hw/qdev-core.h"
>>   
>>   #ifdef CONFIG_NUMA
>>   #include <numaif.h>
>> @@ -237,7 +238,7 @@ static void host_memory_backend_set_prealloc(Object *obj, bool value,
>>           uint64_t sz = memory_region_size(&backend->mr);
>>   
>>           if (!qemu_prealloc_mem(fd, ptr, sz, backend->prealloc_threads,
>> -                               backend->prealloc_context, errp)) {
>> +                               backend->prealloc_context, false, errp)) {
>>               return;
>>           }
>>           backend->prealloc = true;
>> @@ -323,6 +324,7 @@ host_memory_backend_memory_complete(UserCreatable *uc, Error **errp)
>>       HostMemoryBackendClass *bc = MEMORY_BACKEND_GET_CLASS(uc);
>>       void *ptr;
>>       uint64_t sz;
>> +    bool async = !phase_check(PHASE_LATE_BACKENDS_CREATED);
>>   
>>       if (!bc->alloc) {
>>           return;
>> @@ -398,7 +400,8 @@ host_memory_backend_memory_complete(UserCreatable *uc, Error **errp)
>>       if (backend->prealloc && !qemu_prealloc_mem(memory_region_get_fd(&backend->mr),
>>                                                   ptr, sz,
>>                                                   backend->prealloc_threads,
>> -                                                backend->prealloc_context, errp)) {
>> +                                                backend->prealloc_context,
>> +                                                async, errp)) {
>>           return;
>>       }
>>   }
>> diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
>> index 99ab989852..ffd119ebac 100644
>> --- a/hw/virtio/virtio-mem.c
>> +++ b/hw/virtio/virtio-mem.c
>> @@ -605,7 +605,7 @@ static int virtio_mem_set_block_state(VirtIOMEM *vmem, uint64_t start_gpa,
>>           int fd = memory_region_get_fd(&vmem->memdev->mr);
>>           Error *local_err = NULL;
>>   
>> -        if (!qemu_prealloc_mem(fd, area, size, 1, NULL, &local_err)) {
>> +        if (!qemu_prealloc_mem(fd, area, size, 1, NULL, false, &local_err)) {
>>               static bool warned;
>>   
>>               /*
>> @@ -1248,7 +1248,7 @@ static int virtio_mem_prealloc_range_cb(VirtIOMEM *vmem, void *arg,
>>       int fd = memory_region_get_fd(&vmem->memdev->mr);
>>       Error *local_err = NULL;
>>   
>> -    if (!qemu_prealloc_mem(fd, area, size, 1, NULL, &local_err)) {
>> +    if (!qemu_prealloc_mem(fd, area, size, 1, NULL, false, &local_err)) {
>>           error_report_err(local_err);
>>           return -ENOMEM;
>>       }
>> diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
>> index 151d968238..83dd9e2485 100644
>> --- a/include/hw/qdev-core.h
>> +++ b/include/hw/qdev-core.h
>> @@ -1071,6 +1071,11 @@ typedef enum MachineInitPhase {
>>        */
>>       PHASE_ACCEL_CREATED,
>>   
>> +    /*
>> +     * Late backend objects have been created and initialized.
>> +     */
>> +    PHASE_LATE_BACKENDS_CREATED,
>> +
>>       /*
>>        * machine_class->init has been called, thus creating any embedded
>>        * devices and validating machine properties.  Devices created at
>> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
>> index c9692cc314..7d359dabc4 100644
>> --- a/include/qemu/osdep.h
>> +++ b/include/qemu/osdep.h
>> @@ -680,6 +680,8 @@ typedef struct ThreadContext ThreadContext;
>>    * @area: start address of the are to preallocate
>>    * @sz: the size of the area to preallocate
>>    * @max_threads: maximum number of threads to use
>> + * @tc: prealloc context threads pointer, NULL if not in use
>> + * @async: request asynchronous preallocation, requires @tc
>>    * @errp: returns an error if this function fails
>>    *
>>    * Preallocate memory (populate/prefault page tables writable) for the virtual
>> @@ -687,10 +689,24 @@ typedef struct ThreadContext ThreadContext;
>>    * each page in the area was faulted in writable at least once, for example,
>>    * after allocating file blocks for mapped files.
>>    *
>> + * When setting @async, allocation might be performed asynchronously.
>> + * qemu_finish_async_prealloc_mem() must be called to finish any asynchronous
>> + * preallocation.
>> + *
>>    * Return: true on success, else false setting @errp with error.
>>    */
>>   bool qemu_prealloc_mem(int fd, char *area, size_t sz, int max_threads,
>> -                       ThreadContext *tc, Error **errp);
>> +                       ThreadContext *tc, bool async, Error **errp);
>> +
>> +/**
>> + * qemu_finish_async_prealloc_mem:
>> + * @errp: returns an error if this function fails
>> + *
>> + * Finish all outstanding asynchronous memory preallocation.
>> + *
>> + * Return: true on success, else false setting @errp with error.
>> + */
>> +bool qemu_finish_async_prealloc_mem(Error **errp);
>>   
>>   /**
>>    * qemu_get_pid_name:
>> diff --git a/system/vl.c b/system/vl.c
>> index 788d88ea03..e6bc5d9dd9 100644
>> --- a/system/vl.c
>> +++ b/system/vl.c
>> @@ -2009,6 +2009,14 @@ static void qemu_create_late_backends(void)
>>   
>>       object_option_foreach_add(object_create_late);
>>   
>> +    /*
>> +     * Wait for any outstanding memory prealloc from created memory
>> +     * backends to complete.
>> +     */
>> +    if (!qemu_finish_async_prealloc_mem(&error_fatal)) {
>> +        exit(1);
>> +    }
>> +
>>       if (tpm_init() < 0) {
>>           exit(1);
>>       }
>> @@ -3695,6 +3703,7 @@ void qemu_init(int argc, char **argv)
>>        * over memory-backend-file objects).
>>        */
>>       qemu_create_late_backends();
>> +    phase_advance(PHASE_LATE_BACKENDS_CREATED);
>>   
>>       /*
>>        * Note: creates a QOM object, must run only after global and
>> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
>> index 7c297003b9..dada4722f6 100644
>> --- a/util/oslib-posix.c
>> +++ b/util/oslib-posix.c
>> @@ -42,6 +42,7 @@
>>   #include "qemu/cutils.h"
>>   #include "qemu/units.h"
>>   #include "qemu/thread-context.h"
>> +#include "qemu/main-loop.h"
>>   
>>   #ifdef CONFIG_LINUX
>>   #include <sys/syscall.h>
>> @@ -63,11 +64,15 @@
>>   
>>   struct MemsetThread;
>>   
>> +static QLIST_HEAD(, MemsetContext) memset_contexts =
>> +    QLIST_HEAD_INITIALIZER(memset_contexts);
>> +
>>   typedef struct MemsetContext {
>>       bool all_threads_created;
>>       bool any_thread_failed;
>>       struct MemsetThread *threads;
>>       int num_threads;
>> +    QLIST_ENTRY(MemsetContext) next;
>>   } MemsetContext;
>>   
>>   struct MemsetThread {
>> @@ -412,19 +417,44 @@ static inline int get_memset_num_threads(size_t hpagesize, size_t numpages,
>>       return ret;
>>   }
>>   
>> +static int wait_and_free_mem_prealloc_context(MemsetContext *context)
>> +{
>> +    int i, ret = 0, tmp;
>> +
>> +    for (i = 0; i < context->num_threads; i++) {
>> +        tmp = (uintptr_t)qemu_thread_join(&context->threads[i].pgthread);
>> +
>> +        if (tmp) {
>> +            ret = tmp;
>> +        }
>> +    }
>> +    g_free(context->threads);
>> +    g_free(context);
>> +    return ret;
>> +}
>> +
>>   static int touch_all_pages(char *area, size_t hpagesize, size_t numpages,
>> -                           int max_threads, ThreadContext *tc,
>> +                           int max_threads, ThreadContext *tc, bool async,
>>                              bool use_madv_populate_write)
>>   {
>>       static gsize initialized = 0;
>> -    MemsetContext context = {
>> -        .num_threads = get_memset_num_threads(hpagesize, numpages, max_threads),
>> -    };
>> +    MemsetContext *context = g_malloc0(sizeof(MemsetContext));
>>       size_t numpages_per_thread, leftover;
>>       void *(*touch_fn)(void *);
>> -    int ret = 0, i = 0;
>> +    int ret, i = 0;
>>       char *addr = area;
>>   
>> +    /*
>> +     * Asynchronous preallocation is only allowed when using MADV_POPULATE_WRITE
>> +     * and prealloc context for thread placement.
>> +     */
>> +    if (!use_madv_populate_write || !tc) {
>> +        async = false;
>> +    }
>> +
>> +    context->num_threads =
>> +        get_memset_num_threads(hpagesize, numpages, max_threads);
>> +
>>       if (g_once_init_enter(&initialized)) {
>>           qemu_mutex_init(&page_mutex);
>>           qemu_cond_init(&page_cond);
>> @@ -432,8 +462,11 @@ static int touch_all_pages(char *area, size_t hpagesize, size_t numpages,
>>       }
>>   
>>       if (use_madv_populate_write) {
>> -        /* Avoid creating a single thread for MADV_POPULATE_WRITE */
>> -        if (context.num_threads == 1) {
>> +        /*
>> +         * Avoid creating a single thread for MADV_POPULATE_WRITE when
>> +         * preallocating synchronously.
>> +         */
>> +        if (context->num_threads == 1 && !async) {
>>               if (qemu_madvise(area, hpagesize * numpages,
>>                                QEMU_MADV_POPULATE_WRITE)) {
>>                   return -errno;
>> @@ -445,50 +478,86 @@ static int touch_all_pages(char *area, size_t hpagesize, size_t numpages,
>>           touch_fn = do_touch_pages;
>>       }
>>   
>> -    context.threads = g_new0(MemsetThread, context.num_threads);
>> -    numpages_per_thread = numpages / context.num_threads;
>> -    leftover = numpages % context.num_threads;
>> -    for (i = 0; i < context.num_threads; i++) {
>> -        context.threads[i].addr = addr;
>> -        context.threads[i].numpages = numpages_per_thread + (i < leftover);
>> -        context.threads[i].hpagesize = hpagesize;
>> -        context.threads[i].context = &context;
>> +    context->threads = g_new0(MemsetThread, context->num_threads);
>> +    numpages_per_thread = numpages / context->num_threads;
>> +    leftover = numpages % context->num_threads;
>> +    for (i = 0; i < context->num_threads; i++) {
>> +        context->threads[i].addr = addr;
>> +        context->threads[i].numpages = numpages_per_thread + (i < leftover);
>> +        context->threads[i].hpagesize = hpagesize;
>> +        context->threads[i].context = context;
>>           if (tc) {
>> -            thread_context_create_thread(tc, &context.threads[i].pgthread,
>> +            thread_context_create_thread(tc, &context->threads[i].pgthread,
>>                                            "touch_pages",
>> -                                         touch_fn, &context.threads[i],
>> +                                         touch_fn, &context->threads[i],
>>                                            QEMU_THREAD_JOINABLE);
>>           } else {
>> -            qemu_thread_create(&context.threads[i].pgthread, "touch_pages",
>> -                               touch_fn, &context.threads[i],
>> +            qemu_thread_create(&context->threads[i].pgthread, "touch_pages",
>> +                               touch_fn, &context->threads[i],
>>                                  QEMU_THREAD_JOINABLE);
>>           }
>> -        addr += context.threads[i].numpages * hpagesize;
>> +        addr += context->threads[i].numpages * hpagesize;
>> +    }
>> +
>> +    if (async) {
>> +        /*
>> +         * async requests currently require the BQL. Add it to the list and kick
>> +         * preallocation off during qemu_finish_async_prealloc_mem().
>> +         */
>> +        assert(bql_locked());
>> +        QLIST_INSERT_HEAD(&memset_contexts, context, next);
>> +        return 0;
>>       }
>>   
>>       if (!use_madv_populate_write) {
>> -        sigbus_memset_context = &context;
>> +        sigbus_memset_context = context;
>>       }
>>   
>>       qemu_mutex_lock(&page_mutex);
>> -    context.all_threads_created = true;
>> +    context->all_threads_created = true;
>>       qemu_cond_broadcast(&page_cond);
>>       qemu_mutex_unlock(&page_mutex);
>>   
>> -    for (i = 0; i < context.num_threads; i++) {
>> -        int tmp = (uintptr_t)qemu_thread_join(&context.threads[i].pgthread);
>> +    ret = wait_and_free_mem_prealloc_context(context);
>>   
>> +    if (!use_madv_populate_write) {
>> +        sigbus_memset_context = NULL;
>> +    }
>> +    return ret;
>> +}
>> +
>> +bool qemu_finish_async_prealloc_mem(Error **errp)
>> +{
>> +    int ret, tmp;
> 
> The above should be initialized?
> 
> I did a build test and encounter:
> 
> In file included from ../util/oslib-posix.c:36:
> ../util/oslib-posix.c: In function ‘qemu_finish_async_prealloc_mem’:
> /home/libvirt/vm/software/qemu/include/qapi/error.h:334:5: error: ‘ret’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
>    334 |     error_setg_errno_internal((errp), __FILE__, __LINE__, __func__,     \
>        |     ^~~~~~~~~~~~~~~~~~~~~~~~~
> ../util/oslib-posix.c:531:9: note: ‘ret’ was declared here
>    531 |     int ret, tmp;
>        |         ^~~
> cc1: all warnings being treated as errors
> ninja: build stopped: subcommand failed.
> make: *** [Makefile:162: run-ninja] Error 1

Thanks! It's a false-positive, ret could only be initialized if there 
are no contexts, but we check that right at the beginning of the function.

My compiler seems to realize that and does not complain.

I fixed that up and queued the patch to

https://github.com/davidhildenbrand/qemu.git mem-next

If there are no more comments, I'm planning on sending a patch later 
this week.