* David Hildenbrand ([email protected]) wrote: > Resizing while migrating is dangerous and does not work as expected. > The whole migration code works on the usable_length of ram blocks and does > not expect this to change at random points in time. > > Precopy: The ram block size must not change on the source, after > ram_save_setup(), so as long as the guest is still running on the source. > > Postcopy: The ram block size must not change on the target, after > synchronizing the RAM block list (ram_load_precopy()). > > AFAIKS, resizing can be trigger *after* (but not during) a reset in > ACPI code by the guest > - hw/arm/virt-acpi-build.c:acpi_ram_update() > - hw/i386/acpi-build.c:acpi_ram_update() > > I see no easy way to work around this. Fail hard instead of failing > somewhere in migration code due to strange other reasons. AFAIKs, the > rebuilts will be triggered during reboot, so this should not affect > running guests, but only guests that reboot at a very bad time and > actually require size changes. > > Let's further limit the impact by checking if an actual resize of the > RAM (in number of pages) is required. > > Don't perform the checks in qemu_ram_resize(), as that's called during > migration when syncing the used_length. Update documentation.
Interesting; we need to do something about this - but banning resets during migration is a bit harsh; and aborting the source VM is really nasty - for a precopy especially we shouldn't kill the source VM, we should just abort the migration. The other thing that worries me is that acpi_build_update calls acpi_ram_update->memory_region_ram_resize multiple times. So, it might be that the size you end up with at the end of acpi_build_update is actually the same size as the original - so the net effect is the RAMBlock didn't really get resized. Dave > Cc: "Dr. David Alan Gilbert" <[email protected]> > Cc: Eduardo Habkost <[email protected]> > Cc: Paolo Bonzini <[email protected]> > Cc: Igor Mammedov <[email protected]> > Cc: "Michael S. Tsirkin" <[email protected]> > Cc: Richard Henderson <[email protected]> > Cc: Shannon Zhao <[email protected]> > Cc: Alex Bennée <[email protected]> > Cc: Shameerali Kolothum Thodi <[email protected]> > Cc: Juan Quintela <[email protected]> > Signed-off-by: David Hildenbrand <[email protected]> > --- > > Any idea how to avoid killing the guest? Anything obvious I am missing? > > --- > exec.c | 6 ++++-- > include/exec/memory.h | 11 +++++++---- > memory.c | 12 ++++++++++++ > 3 files changed, 23 insertions(+), 6 deletions(-) > > diff --git a/exec.c b/exec.c > index 67e520d18e..faa6708414 100644 > --- a/exec.c > +++ b/exec.c > @@ -2116,8 +2116,10 @@ static int memory_try_enable_merging(void *addr, > size_t len) > return qemu_madvise(addr, len, QEMU_MADV_MERGEABLE); > } > > -/* Only legal before guest might have detected the memory size: e.g. on > - * incoming migration, or right after reset. > +/* > + * RAM must not be resized while migration is active (except from migration > + * code). Care has to be taken if the guest might have already detected > + * the memory. > * > * As memory core doesn't know how is memory accessed, it is up to > * resize callback to update device state and/or add assertions to detect > diff --git a/include/exec/memory.h b/include/exec/memory.h > index e85b7de99a..1e5bfbe805 100644 > --- a/include/exec/memory.h > +++ b/include/exec/memory.h > @@ -113,7 +113,8 @@ typedef struct IOMMUNotifier IOMMUNotifier; > #define RAM_SHARED (1 << 1) > > /* Only a portion of RAM (used_length) is actually used, and migrated. > - * This used_length size can change across reboots. > + * RAM must not be resized while migration is active (except from migration > + * code). > */ > #define RAM_RESIZEABLE (1 << 2) > > @@ -843,7 +844,8 @@ void memory_region_init_ram_shared_nomigrate(MemoryRegion > *mr, > * RAM. Accesses into the region will > * modify memory directly. Only an > initial > * portion of this RAM is actually used. > - * The used size can change across > reboots. > + * The size must not change while > migration > + * is active. > * > * @mr: the #MemoryRegion to be initialized. > * @owner: the object that tracks the region's reference count > @@ -1464,8 +1466,9 @@ void *memory_region_get_ram_ptr(MemoryRegion *mr); > > /* memory_region_ram_resize: Resize a RAM region. > * > - * Only legal before guest might have detected the memory size: e.g. on > - * incoming migration, or right after reset. > + * RAM must not be resized while migration is active (except from migration > + * code). Care has to be taken if the guest might have already detected > + * the memory. > * > * @mr: a memory region created with @memory_region_init_resizeable_ram. > * @newsize: the new size the region > diff --git a/memory.c b/memory.c > index aeaa8dcc9e..7fa048aa3a 100644 > --- a/memory.c > +++ b/memory.c > @@ -34,6 +34,7 @@ > #include "sysemu/accel.h" > #include "hw/boards.h" > #include "migration/vmstate.h" > +#include "migration/misc.h" > > //#define DEBUG_UNASSIGNED > > @@ -2204,6 +2205,17 @@ void memory_region_ram_resize(MemoryRegion *mr, > ram_addr_t newsize, Error **errp > { > assert(mr->ram_block); > > + /* > + * Resizing RAM while migrating is not possible, as the used_length of > + * RAM blocks must neither change on the source (precopy), nor on the > + * target (postcopy) as long as migration code is active. > + */ > + if (HOST_PAGE_ALIGN(newsize) != mr->ram_block->used_length && > + !migration_is_idle()) { > + error_setg(errp, "Cannot resize RAM while migrating."); > + return; > + } > + > qemu_ram_resize(mr->ram_block, newsize, errp); > } > > -- > 2.24.1 > -- Dr. David Alan Gilbert / [email protected] / Manchester, UK
