| Message ID | 20220216062809.57179-5-peterx@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | migration: Postcopy Preemption | expand |
* Peter Xu (peterx@redhat.com) wrote: > Postcopy handles huge pages in a special way that currently we can only have > one "channel" to transfer the page. > > It's because when we install pages using UFFDIO_COPY, we need to have the whole > huge page ready, it also means we need to have a temp huge page when trying to > receive the whole content of the page. > > Currently all maintainance around this tmp page is global: firstly we'll > allocate a temp huge page, then we maintain its status mostly within > ram_load_postcopy(). > > To enable multiple channels for postcopy, the first thing we need to do is to > prepare N temp huge pages as caching, one for each channel. > > Meanwhile we need to maintain the tmp huge page status per-channel too. > > To give some example, some local variables maintained in ram_load_postcopy() > are listed; they are responsible for maintaining temp huge page status: > > - all_zero: this keeps whether this huge page contains all zeros > - target_pages: this counts how many target pages have been copied > - host_page: this keeps the host ptr for the page to install > > Move all these fields to be together with the temp huge pages to form a new > structure called PostcopyTmpPage. Then for each (future) postcopy channel, we > need one structure to keep the state around. > > For vanilla postcopy, obviously there's only one channel. It contains both > precopy and postcopy pages. > > This patch teaches the dest migration node to start realize the possible number > of postcopy channels by introducing the "postcopy_channels" variable. Its > value is calculated when setup postcopy on dest node (during POSTCOPY_LISTEN > phase). > > Vanilla postcopy will have channels=1, but when postcopy-preempt capability is > enabled (in the future), we will boost it to 2 because even during partial > sending of a precopy huge page we still want to preempt it and start sending > the postcopy requested page right away (so we start to keep two temp huge > pages; more if we want to enable multifd). In this patch there's a TODO marked > for that; so far the channels is always set to 1. > > We need to send one "host huge page" on one channel only and we cannot split > them, because otherwise the data upon the same huge page can locate on more > than one channel so we need more complicated logic to manage. One temp host > huge page for each channel will be enough for us for now. > > Postcopy will still always use the index=0 huge page even after this patch. > However it prepares for the latter patches where it can start to use multiple > channels (which needs src intervention, because only src knows which channel we > should use). > > Signed-off-by: Peter Xu <peterx@redhat.com> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com> > --- > migration/migration.h | 36 +++++++++++++++++++++++- > migration/postcopy-ram.c | 60 ++++++++++++++++++++++++++++++---------- > migration/ram.c | 43 ++++++++++++++-------------- > migration/savevm.c | 12 ++++++++ > 4 files changed, 113 insertions(+), 38 deletions(-) > > diff --git a/migration/migration.h b/migration/migration.h > index 8130b703eb..42c7395094 100644 > --- a/migration/migration.h > +++ b/migration/migration.h > @@ -45,6 +45,24 @@ struct PostcopyBlocktimeContext; > */ > #define CLEAR_BITMAP_SHIFT_MAX 31 > > +/* This is an abstraction of a "temp huge page" for postcopy's purpose */ > +typedef struct { > + /* > + * This points to a temporary huge page as a buffer for UFFDIO_COPY. It's > + * mmap()ed and needs to be freed when cleanup. > + */ > + void *tmp_huge_page; > + /* > + * This points to the host page we're going to install for this temp page. > + * It tells us after we've received the whole page, where we should put it. > + */ > + void *host_addr; > + /* Number of small pages copied (in size of TARGET_PAGE_SIZE) */ > + unsigned int target_pages; > + /* Whether this page contains all zeros */ > + bool all_zero; > +} PostcopyTmpPage; > + > /* State for the incoming migration */ > struct MigrationIncomingState { > QEMUFile *from_src_file; > @@ -81,7 +99,22 @@ struct MigrationIncomingState { > QemuMutex rp_mutex; /* We send replies from multiple threads */ > /* RAMBlock of last request sent to source */ > RAMBlock *last_rb; > - void *postcopy_tmp_page; > + /* > + * Number of postcopy channels including the default precopy channel, so > + * vanilla postcopy will only contain one channel which contain both > + * precopy and postcopy streams. > + * > + * This is calculated when the src requests to enable postcopy but before > + * it starts. Its value can depend on e.g. whether postcopy preemption is > + * enabled. > + */ > + unsigned int postcopy_channels; > + /* > + * An array of temp host huge pages to be used, one for each postcopy > + * channel. > + */ > + PostcopyTmpPage *postcopy_tmp_pages; > + /* This is shared for all postcopy channels */ > void *postcopy_tmp_zero_page; > /* PostCopyFD's for external userfaultfds & handlers of shared memory */ > GArray *postcopy_remote_fds; > @@ -391,5 +424,6 @@ bool migration_rate_limit(void); > void migration_cancel(const Error *error); > > void populate_vfio_info(MigrationInfo *info); > +void postcopy_temp_page_reset(PostcopyTmpPage *tmp_page); > > #endif > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c > index e662dd05cc..315f784965 100644 > --- a/migration/postcopy-ram.c > +++ b/migration/postcopy-ram.c > @@ -525,9 +525,18 @@ int postcopy_ram_incoming_init(MigrationIncomingState *mis) > > static void postcopy_temp_pages_cleanup(MigrationIncomingState *mis) > { > - if (mis->postcopy_tmp_page) { > - munmap(mis->postcopy_tmp_page, mis->largest_page_size); > - mis->postcopy_tmp_page = NULL; > + int i; > + > + if (mis->postcopy_tmp_pages) { > + for (i = 0; i < mis->postcopy_channels; i++) { > + if (mis->postcopy_tmp_pages[i].tmp_huge_page) { > + munmap(mis->postcopy_tmp_pages[i].tmp_huge_page, > + mis->largest_page_size); > + mis->postcopy_tmp_pages[i].tmp_huge_page = NULL; > + } > + } > + g_free(mis->postcopy_tmp_pages); > + mis->postcopy_tmp_pages = NULL; > } > > if (mis->postcopy_tmp_zero_page) { > @@ -1091,17 +1100,30 @@ retry: > > static int postcopy_temp_pages_setup(MigrationIncomingState *mis) > { > - int err; > - > - mis->postcopy_tmp_page = mmap(NULL, mis->largest_page_size, > - PROT_READ | PROT_WRITE, > - MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); > - if (mis->postcopy_tmp_page == MAP_FAILED) { > - err = errno; > - mis->postcopy_tmp_page = NULL; > - error_report("%s: Failed to map postcopy_tmp_page %s", > - __func__, strerror(err)); > - return -err; > + PostcopyTmpPage *tmp_page; > + int err, i, channels; > + void *temp_page; > + > + /* TODO: will be boosted when enable postcopy preemption */ > + mis->postcopy_channels = 1; > + > + channels = mis->postcopy_channels; > + mis->postcopy_tmp_pages = g_malloc0_n(sizeof(PostcopyTmpPage), channels); > + > + for (i = 0; i < channels; i++) { > + tmp_page = &mis->postcopy_tmp_pages[i]; > + temp_page = mmap(NULL, mis->largest_page_size, PROT_READ | PROT_WRITE, > + MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); > + if (temp_page == MAP_FAILED) { > + err = errno; > + error_report("%s: Failed to map postcopy_tmp_pages[%d]: %s", > + __func__, i, strerror(err)); > + /* Clean up will be done later */ > + return -err; > + } > + tmp_page->tmp_huge_page = temp_page; > + /* Initialize default states for each tmp page */ > + postcopy_temp_page_reset(tmp_page); > } > > /* > @@ -1351,6 +1373,16 @@ int postcopy_wake_shared(struct PostCopyFD *pcfd, > #endif > > /* ------------------------------------------------------------------------- */ > +void postcopy_temp_page_reset(PostcopyTmpPage *tmp_page) > +{ > + tmp_page->target_pages = 0; > + tmp_page->host_addr = NULL; > + /* > + * This is set to true when reset, and cleared as long as we received any > + * of the non-zero small page within this huge page. > + */ > + tmp_page->all_zero = true; > +} > > void postcopy_fault_thread_notify(MigrationIncomingState *mis) > { > diff --git a/migration/ram.c b/migration/ram.c > index 91ca743ac8..36b0a53afe 100644 > --- a/migration/ram.c > +++ b/migration/ram.c > @@ -3640,11 +3640,8 @@ static int ram_load_postcopy(QEMUFile *f) > bool place_needed = false; > bool matches_target_page_size = false; > MigrationIncomingState *mis = migration_incoming_get_current(); > - /* Temporary page that is later 'placed' */ > - void *postcopy_host_page = mis->postcopy_tmp_page; > - void *host_page = NULL; > - bool all_zero = true; > - int target_pages = 0; > + /* Currently we only use channel 0. TODO: use all the channels */ > + PostcopyTmpPage *tmp_page = &mis->postcopy_tmp_pages[0]; > > while (!ret && !(flags & RAM_SAVE_FLAG_EOS)) { > ram_addr_t addr; > @@ -3688,7 +3685,7 @@ static int ram_load_postcopy(QEMUFile *f) > ret = -EINVAL; > break; > } > - target_pages++; > + tmp_page->target_pages++; > matches_target_page_size = block->page_size == TARGET_PAGE_SIZE; > /* > * Postcopy requires that we place whole host pages atomically; > @@ -3700,15 +3697,16 @@ static int ram_load_postcopy(QEMUFile *f) > * however the source ensures it always sends all the components > * of a host page in one chunk. > */ > - page_buffer = postcopy_host_page + > + page_buffer = tmp_page->tmp_huge_page + > host_page_offset_from_ram_block_offset(block, addr); > /* If all TP are zero then we can optimise the place */ > - if (target_pages == 1) { > - host_page = host_page_from_ram_block_offset(block, addr); > - } else if (host_page != host_page_from_ram_block_offset(block, > - addr)) { > + if (tmp_page->target_pages == 1) { > + tmp_page->host_addr = > + host_page_from_ram_block_offset(block, addr); > + } else if (tmp_page->host_addr != > + host_page_from_ram_block_offset(block, addr)) { > /* not the 1st TP within the HP */ > - error_report("Non-same host page %p/%p", host_page, > + error_report("Non-same host page %p/%p", tmp_page->host_addr, > host_page_from_ram_block_offset(block, addr)); > ret = -EINVAL; > break; > @@ -3718,10 +3716,11 @@ static int ram_load_postcopy(QEMUFile *f) > * If it's the last part of a host page then we place the host > * page > */ > - if (target_pages == (block->page_size / TARGET_PAGE_SIZE)) { > + if (tmp_page->target_pages == > + (block->page_size / TARGET_PAGE_SIZE)) { > place_needed = true; > } > - place_source = postcopy_host_page; > + place_source = tmp_page->tmp_huge_page; > } > > switch (flags & ~RAM_SAVE_FLAG_CONTINUE) { > @@ -3735,12 +3734,12 @@ static int ram_load_postcopy(QEMUFile *f) > memset(page_buffer, ch, TARGET_PAGE_SIZE); > } > if (ch) { > - all_zero = false; > + tmp_page->all_zero = false; > } > break; > > case RAM_SAVE_FLAG_PAGE: > - all_zero = false; > + tmp_page->all_zero = false; > if (!matches_target_page_size) { > /* For huge pages, we always use temporary buffer */ > qemu_get_buffer(f, page_buffer, TARGET_PAGE_SIZE); > @@ -3758,7 +3757,7 @@ static int ram_load_postcopy(QEMUFile *f) > } > break; > case RAM_SAVE_FLAG_COMPRESS_PAGE: > - all_zero = false; > + tmp_page->all_zero = false; > len = qemu_get_be32(f); > if (len < 0 || len > compressBound(TARGET_PAGE_SIZE)) { > error_report("Invalid compressed data length: %d", len); > @@ -3790,16 +3789,14 @@ static int ram_load_postcopy(QEMUFile *f) > } > > if (!ret && place_needed) { > - if (all_zero) { > - ret = postcopy_place_page_zero(mis, host_page, block); > + if (tmp_page->all_zero) { > + ret = postcopy_place_page_zero(mis, tmp_page->host_addr, block); > } else { > - ret = postcopy_place_page(mis, host_page, place_source, > + ret = postcopy_place_page(mis, tmp_page->host_addr, place_source, > block); > } > place_needed = false; > - target_pages = 0; > - /* Assume we have a zero page until we detect something different */ > - all_zero = true; > + postcopy_temp_page_reset(tmp_page); > } > } > > diff --git a/migration/savevm.c b/migration/savevm.c > index 41e3238798..0ccd7e5e3f 100644 > --- a/migration/savevm.c > +++ b/migration/savevm.c > @@ -2579,6 +2579,18 @@ void qemu_loadvm_state_cleanup(void) > /* Return true if we should continue the migration, or false. */ > static bool postcopy_pause_incoming(MigrationIncomingState *mis) > { > + int i; > + > + /* > + * If network is interrupted, any temp page we received will be useless > + * because we didn't mark them as "received" in receivedmap. After a > + * proper recovery later (which will sync src dirty bitmap with receivedmap > + * on dest) these cached small pages will be resent again. > + */ > + for (i = 0; i < mis->postcopy_channels; i++) { > + postcopy_temp_page_reset(&mis->postcopy_tmp_pages[i]); > + } > + > trace_postcopy_pause_incoming(); > > assert(migrate_postcopy_ram()); > -- > 2.32.0 >
diff --git a/migration/migration.h b/migration/migration.h index 8130b703eb..42c7395094 100644 --- a/migration/migration.h +++ b/migration/migration.h @@ -45,6 +45,24 @@ struct PostcopyBlocktimeContext; */ #define CLEAR_BITMAP_SHIFT_MAX 31 +/* This is an abstraction of a "temp huge page" for postcopy's purpose */ +typedef struct { + /* + * This points to a temporary huge page as a buffer for UFFDIO_COPY. It's + * mmap()ed and needs to be freed when cleanup. + */ + void *tmp_huge_page; + /* + * This points to the host page we're going to install for this temp page. + * It tells us after we've received the whole page, where we should put it. + */ + void *host_addr; + /* Number of small pages copied (in size of TARGET_PAGE_SIZE) */ + unsigned int target_pages; + /* Whether this page contains all zeros */ + bool all_zero; +} PostcopyTmpPage; + /* State for the incoming migration */ struct MigrationIncomingState { QEMUFile *from_src_file; @@ -81,7 +99,22 @@ struct MigrationIncomingState { QemuMutex rp_mutex; /* We send replies from multiple threads */ /* RAMBlock of last request sent to source */ RAMBlock *last_rb; - void *postcopy_tmp_page; + /* + * Number of postcopy channels including the default precopy channel, so + * vanilla postcopy will only contain one channel which contain both + * precopy and postcopy streams. + * + * This is calculated when the src requests to enable postcopy but before + * it starts. Its value can depend on e.g. whether postcopy preemption is + * enabled. + */ + unsigned int postcopy_channels; + /* + * An array of temp host huge pages to be used, one for each postcopy + * channel. + */ + PostcopyTmpPage *postcopy_tmp_pages; + /* This is shared for all postcopy channels */ void *postcopy_tmp_zero_page; /* PostCopyFD's for external userfaultfds & handlers of shared memory */ GArray *postcopy_remote_fds; @@ -391,5 +424,6 @@ bool migration_rate_limit(void); void migration_cancel(const Error *error); void populate_vfio_info(MigrationInfo *info); +void postcopy_temp_page_reset(PostcopyTmpPage *tmp_page); #endif diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c index e662dd05cc..315f784965 100644 --- a/migration/postcopy-ram.c +++ b/migration/postcopy-ram.c @@ -525,9 +525,18 @@ int postcopy_ram_incoming_init(MigrationIncomingState *mis) static void postcopy_temp_pages_cleanup(MigrationIncomingState *mis) { - if (mis->postcopy_tmp_page) { - munmap(mis->postcopy_tmp_page, mis->largest_page_size); - mis->postcopy_tmp_page = NULL; + int i; + + if (mis->postcopy_tmp_pages) { + for (i = 0; i < mis->postcopy_channels; i++) { + if (mis->postcopy_tmp_pages[i].tmp_huge_page) { + munmap(mis->postcopy_tmp_pages[i].tmp_huge_page, + mis->largest_page_size); + mis->postcopy_tmp_pages[i].tmp_huge_page = NULL; + } + } + g_free(mis->postcopy_tmp_pages); + mis->postcopy_tmp_pages = NULL; } if (mis->postcopy_tmp_zero_page) { @@ -1091,17 +1100,30 @@ retry: static int postcopy_temp_pages_setup(MigrationIncomingState *mis) { - int err; - - mis->postcopy_tmp_page = mmap(NULL, mis->largest_page_size, - PROT_READ | PROT_WRITE, - MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); - if (mis->postcopy_tmp_page == MAP_FAILED) { - err = errno; - mis->postcopy_tmp_page = NULL; - error_report("%s: Failed to map postcopy_tmp_page %s", - __func__, strerror(err)); - return -err; + PostcopyTmpPage *tmp_page; + int err, i, channels; + void *temp_page; + + /* TODO: will be boosted when enable postcopy preemption */ + mis->postcopy_channels = 1; + + channels = mis->postcopy_channels; + mis->postcopy_tmp_pages = g_malloc0_n(sizeof(PostcopyTmpPage), channels); + + for (i = 0; i < channels; i++) { + tmp_page = &mis->postcopy_tmp_pages[i]; + temp_page = mmap(NULL, mis->largest_page_size, PROT_READ | PROT_WRITE, + MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); + if (temp_page == MAP_FAILED) { + err = errno; + error_report("%s: Failed to map postcopy_tmp_pages[%d]: %s", + __func__, i, strerror(err)); + /* Clean up will be done later */ + return -err; + } + tmp_page->tmp_huge_page = temp_page; + /* Initialize default states for each tmp page */ + postcopy_temp_page_reset(tmp_page); } /* @@ -1351,6 +1373,16 @@ int postcopy_wake_shared(struct PostCopyFD *pcfd, #endif /* ------------------------------------------------------------------------- */ +void postcopy_temp_page_reset(PostcopyTmpPage *tmp_page) +{ + tmp_page->target_pages = 0; + tmp_page->host_addr = NULL; + /* + * This is set to true when reset, and cleared as long as we received any + * of the non-zero small page within this huge page. + */ + tmp_page->all_zero = true; +} void postcopy_fault_thread_notify(MigrationIncomingState *mis) { diff --git a/migration/ram.c b/migration/ram.c index 91ca743ac8..36b0a53afe 100644 --- a/migration/ram.c +++ b/migration/ram.c @@ -3640,11 +3640,8 @@ static int ram_load_postcopy(QEMUFile *f) bool place_needed = false; bool matches_target_page_size = false; MigrationIncomingState *mis = migration_incoming_get_current(); - /* Temporary page that is later 'placed' */ - void *postcopy_host_page = mis->postcopy_tmp_page; - void *host_page = NULL; - bool all_zero = true; - int target_pages = 0; + /* Currently we only use channel 0. TODO: use all the channels */ + PostcopyTmpPage *tmp_page = &mis->postcopy_tmp_pages[0]; while (!ret && !(flags & RAM_SAVE_FLAG_EOS)) { ram_addr_t addr; @@ -3688,7 +3685,7 @@ static int ram_load_postcopy(QEMUFile *f) ret = -EINVAL; break; } - target_pages++; + tmp_page->target_pages++; matches_target_page_size = block->page_size == TARGET_PAGE_SIZE; /* * Postcopy requires that we place whole host pages atomically; @@ -3700,15 +3697,16 @@ static int ram_load_postcopy(QEMUFile *f) * however the source ensures it always sends all the components * of a host page in one chunk. */ - page_buffer = postcopy_host_page + + page_buffer = tmp_page->tmp_huge_page + host_page_offset_from_ram_block_offset(block, addr); /* If all TP are zero then we can optimise the place */ - if (target_pages == 1) { - host_page = host_page_from_ram_block_offset(block, addr); - } else if (host_page != host_page_from_ram_block_offset(block, - addr)) { + if (tmp_page->target_pages == 1) { + tmp_page->host_addr = + host_page_from_ram_block_offset(block, addr); + } else if (tmp_page->host_addr != + host_page_from_ram_block_offset(block, addr)) { /* not the 1st TP within the HP */ - error_report("Non-same host page %p/%p", host_page, + error_report("Non-same host page %p/%p", tmp_page->host_addr, host_page_from_ram_block_offset(block, addr)); ret = -EINVAL; break; @@ -3718,10 +3716,11 @@ static int ram_load_postcopy(QEMUFile *f) * If it's the last part of a host page then we place the host * page */ - if (target_pages == (block->page_size / TARGET_PAGE_SIZE)) { + if (tmp_page->target_pages == + (block->page_size / TARGET_PAGE_SIZE)) { place_needed = true; } - place_source = postcopy_host_page; + place_source = tmp_page->tmp_huge_page; } switch (flags & ~RAM_SAVE_FLAG_CONTINUE) { @@ -3735,12 +3734,12 @@ static int ram_load_postcopy(QEMUFile *f) memset(page_buffer, ch, TARGET_PAGE_SIZE); } if (ch) { - all_zero = false; + tmp_page->all_zero = false; } break; case RAM_SAVE_FLAG_PAGE: - all_zero = false; + tmp_page->all_zero = false; if (!matches_target_page_size) { /* For huge pages, we always use temporary buffer */ qemu_get_buffer(f, page_buffer, TARGET_PAGE_SIZE); @@ -3758,7 +3757,7 @@ static int ram_load_postcopy(QEMUFile *f) } break; case RAM_SAVE_FLAG_COMPRESS_PAGE: - all_zero = false; + tmp_page->all_zero = false; len = qemu_get_be32(f); if (len < 0 || len > compressBound(TARGET_PAGE_SIZE)) { error_report("Invalid compressed data length: %d", len); @@ -3790,16 +3789,14 @@ static int ram_load_postcopy(QEMUFile *f) } if (!ret && place_needed) { - if (all_zero) { - ret = postcopy_place_page_zero(mis, host_page, block); + if (tmp_page->all_zero) { + ret = postcopy_place_page_zero(mis, tmp_page->host_addr, block); } else { - ret = postcopy_place_page(mis, host_page, place_source, + ret = postcopy_place_page(mis, tmp_page->host_addr, place_source, block); } place_needed = false; - target_pages = 0; - /* Assume we have a zero page until we detect something different */ - all_zero = true; + postcopy_temp_page_reset(tmp_page); } } diff --git a/migration/savevm.c b/migration/savevm.c index 41e3238798..0ccd7e5e3f 100644 --- a/migration/savevm.c +++ b/migration/savevm.c @@ -2579,6 +2579,18 @@ void qemu_loadvm_state_cleanup(void) /* Return true if we should continue the migration, or false. */ static bool postcopy_pause_incoming(MigrationIncomingState *mis) { + int i; + + /* + * If network is interrupted, any temp page we received will be useless + * because we didn't mark them as "received" in receivedmap. After a + * proper recovery later (which will sync src dirty bitmap with receivedmap + * on dest) these cached small pages will be resent again. + */ + for (i = 0; i < mis->postcopy_channels; i++) { + postcopy_temp_page_reset(&mis->postcopy_tmp_pages[i]); + } + trace_postcopy_pause_incoming(); assert(migrate_postcopy_ram());
Postcopy handles huge pages in a special way that currently we can only have one "channel" to transfer the page. It's because when we install pages using UFFDIO_COPY, we need to have the whole huge page ready, it also means we need to have a temp huge page when trying to receive the whole content of the page. Currently all maintainance around this tmp page is global: firstly we'll allocate a temp huge page, then we maintain its status mostly within ram_load_postcopy(). To enable multiple channels for postcopy, the first thing we need to do is to prepare N temp huge pages as caching, one for each channel. Meanwhile we need to maintain the tmp huge page status per-channel too. To give some example, some local variables maintained in ram_load_postcopy() are listed; they are responsible for maintaining temp huge page status: - all_zero: this keeps whether this huge page contains all zeros - target_pages: this counts how many target pages have been copied - host_page: this keeps the host ptr for the page to install Move all these fields to be together with the temp huge pages to form a new structure called PostcopyTmpPage. Then for each (future) postcopy channel, we need one structure to keep the state around. For vanilla postcopy, obviously there's only one channel. It contains both precopy and postcopy pages. This patch teaches the dest migration node to start realize the possible number of postcopy channels by introducing the "postcopy_channels" variable. Its value is calculated when setup postcopy on dest node (during POSTCOPY_LISTEN phase). Vanilla postcopy will have channels=1, but when postcopy-preempt capability is enabled (in the future), we will boost it to 2 because even during partial sending of a precopy huge page we still want to preempt it and start sending the postcopy requested page right away (so we start to keep two temp huge pages; more if we want to enable multifd). In this patch there's a TODO marked for that; so far the channels is always set to 1. We need to send one "host huge page" on one channel only and we cannot split them, because otherwise the data upon the same huge page can locate on more than one channel so we need more complicated logic to manage. One temp host huge page for each channel will be enough for us for now. Postcopy will still always use the index=0 huge page even after this patch. However it prepares for the latter patches where it can start to use multiple channels (which needs src intervention, because only src knows which channel we should use). Signed-off-by: Peter Xu <peterx@redhat.com> --- migration/migration.h | 36 +++++++++++++++++++++++- migration/postcopy-ram.c | 60 ++++++++++++++++++++++++++++++---------- migration/ram.c | 43 ++++++++++++++-------------- migration/savevm.c | 12 ++++++++ 4 files changed, 113 insertions(+), 38 deletions(-)