Message ID | 1362925309-3852-4-git-send-email-i.mitsyanko@gmail.com |
---|---|
State | New |
Headers | show |
On 10 March 2013 22:21, Igor Mitsyanko <i.mitsyanko@gmail.com> wrote: > Registering memory regions using preallocated memory which size is not a multiple of > target page size will result in inconsistency in QEMU memory system. Do not > allow to do that at all by checking for that case (and asserting) in > memory_region_init_ram_ptr(). This is too vague. What exactly is the problem and why can't we just fix the memory system to correctly handle being passed small preallocated memory areas? > --- a/memory.c > +++ b/memory.c > @@ -949,6 +949,7 @@ void memory_region_init_ram_ptr(MemoryRegion *mr, > uint64_t size, > void *ptr) > { > + assert((size & (TARGET_PAGE_SIZE - 1)) == 0); This in particular seems like a bad idea, because TARGET_PAGE_SIZE is a per-CPU thing, and we shouldn't be adding more code to QEMU which will need to be fixed if/when we ever support multiple CPU types in a single binary. (Also TARGET_PAGE_SIZE isn't necessarily what you think it is: for instance on ARM it's actually only 1K even though the standard ARM setup is 4K pages.) thanks -- PMM
On 10.03.2013 18:27, Peter Maydell wrote: > On 10 March 2013 22:21, Igor Mitsyanko <i.mitsyanko@gmail.com> wrote: >> Registering memory regions using preallocated memory which size is not a multiple of >> target page size will result in inconsistency in QEMU memory system. Do not >> allow to do that at all by checking for that case (and asserting) in >> memory_region_init_ram_ptr(). > This is too vague. What exactly is the problem and why can't we > just fix the memory system to correctly handle being passed > small preallocated memory areas? The problem I've personally encountered is the one I described in PATCH 1. When saving a VM state, QEMU is looping forever in ram_save_block() trying to save chipid_and_omr memory region. This is because migration_bitmap_find_and_reset_dirty() works on memory blocks of TARGET_PAGE_SIZE length. There could be other places where problem could occur I think. Its not really related to an actual TARGET_PAGE_SIZE and its length, what important is to be consistent in minimal memory length requirements. For example, when we pass size=1 byte to memory_region_init_ram_ptr(), it sets MemoryRegion::size to 1 byte, but at the same time, it sets corresponding RAMBlock::size to TARGET_PAGE_ALIGN(1). And now we have a situation when some parts of QEMU think that it can access the whole TARGET_PAGE_SIZE region, while we actually allocated only 1 byte for it. Same goes for migration, it operates on TARGET_PAGE_SIZE-length data blocks only. What I mean to say is, it looks like QEMU has an implicit assumption that RAM length should be a multiple of page size length? > >> --- a/memory.c >> +++ b/memory.c >> @@ -949,6 +949,7 @@ void memory_region_init_ram_ptr(MemoryRegion *mr, >> uint64_t size, >> void *ptr) >> { >> + assert((size & (TARGET_PAGE_SIZE - 1)) == 0); > This in particular seems like a bad idea, because TARGET_PAGE_SIZE > is a per-CPU thing, and we shouldn't be adding more code to QEMU which > will need to be fixed if/when we ever support multiple CPU types in > a single binary. (Also TARGET_PAGE_SIZE isn't necessarily what you > think it is: for instance on ARM it's actually only 1K even though > the standard ARM setup is 4K pages.) > > thanks > -- PMM
diff --git a/include/exec/memory.h b/include/exec/memory.h index 2322732..87b9292 100644 --- a/include/exec/memory.h +++ b/include/exec/memory.h @@ -279,7 +279,7 @@ void memory_region_init_ram(MemoryRegion *mr, * * @mr: the #MemoryRegion to be initialized. * @name: the name of the region. - * @size: size of the region. + * @size: size of the region. Must be a multiple of target page size. * @ptr: memory to be mapped; must contain at least @size bytes. */ void memory_region_init_ram_ptr(MemoryRegion *mr, diff --git a/memory.c b/memory.c index 92a2196..15cb47f 100644 --- a/memory.c +++ b/memory.c @@ -949,6 +949,7 @@ void memory_region_init_ram_ptr(MemoryRegion *mr, uint64_t size, void *ptr) { + assert((size & (TARGET_PAGE_SIZE - 1)) == 0); memory_region_init(mr, name, size); mr->ram = true; mr->terminates = true;
Registering memory regions using preallocated memory which size is not a multiple of target page size will result in inconsistency in QEMU memory system. Do not allow to do that at all by checking for that case (and asserting) in memory_region_init_ram_ptr(). Signed-off-by: Igor Mitsyanko <i.mitsyanko@gmail.com> --- include/exec/memory.h | 2 +- memory.c | 1 + 2 files changed, 2 insertions(+), 1 deletions(-)