[PATCH RFC 0/4] vl: Sync dirty bitmap when system resets

[Peter Xu <peterx@redhat.com>]

This RFC series starts from the fact that we will sync dirty bitmap when
removing a memslot for KVM.  IIUC that was majorly to maintain the dirty bitmap
even across a system reboot.

This series wants to move that sync from kvm memslot removal to system reset.

(I still don't know why the reset system will still need to keep the RAM status
 before the reset.  I thought things like kdump might use this to retrieve info
 from previous kernel panic, however IIUC that's not what kdump is doing now.
 Anyway, I'd be more than glad if anyone knows the real scenario behind
 this...)

The current solution (sync at kvm memslot removal) works in most cases, but:

  - it will be merely impossible to work for dirty ring, and,

  - it has an existing flaw on race condition. [1]

So if system reset is the only thing we care here, I'm thinking whether we can
move this sync explicitly to system reset so we do a global sync there instead
of sync every time when memory layout changed and caused memory removals.  I
think it can be more explict to sync during system reset, and also with that
context it will be far easier for kvm dirty ring to provide the same logic.

This is totally RFC because I'm still trying to find whether there will be
other cases besides system reset that we want to keep the dirty bits for a
to-be-removed memslot (real memory removals like unplugging memory shouldn't
matter, because we won't care about the dirty bits if it's never going to be
there anymore, not to mention we won't allow such things during a migration).
So far I don't see any.

I've run some tests either using the old dirty log or dirty ring, with either
some memory load or reboots on the source, and I see no issues so far.

Comments greatly welcomed.  Thanks.

[1] https://lore.kernel.org/qemu-devel/20200327150425.GJ422390@xz-x1/

Peter Xu (4):
  migration: Export migration_bitmap_sync_precopy()
  migration: Introduce migrate_is_precopy()
  vl: Sync dirty bits for system resets during precopy
  kvm: No need to sync dirty bitmap before memslot removal any more

 accel/kvm/kvm-all.c      |  3 ---
 include/migration/misc.h |  2 ++
 migration/migration.c    |  7 +++++++
 migration/ram.c          | 10 +++++-----
 softmmu/vl.c             | 16 ++++++++++++++++
 5 files changed, 30 insertions(+), 8 deletions(-)

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2.24.1