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Re: [PATCH] KVM: use store-release to mark dirty pages as harvested
From: |
Peter Xu |
Subject: |
Re: [PATCH] KVM: use store-release to mark dirty pages as harvested |
Date: |
Fri, 2 Sep 2022 10:10:27 -0400 |
On Fri, Sep 02, 2022 at 02:19:36AM +0200, Paolo Bonzini wrote:
> The following scenario can happen if QEMU sets more RESET flags while
> the KVM_RESET_DIRTY_RINGS ioctl is ongoing on another host CPU:
>
> CPU0 CPU1 CPU2
> ------------------------ ------------------ ------------------------
> fill gfn0
> store-rel flags for gfn0
> fill gfn1
> store-rel flags for gfn1
> load-acq flags for gfn0
> set RESET for gfn0
> load-acq flags for gfn1
> set RESET for gfn1
> do ioctl! ----------->
> ioctl(RESET_RINGS)
> fill gfn2
> store-rel flags for gfn2
> load-acq flags for gfn2
> set RESET for gfn2
> process gfn0
> process gfn1
> process gfn2
> do ioctl!
> etc.
>
> The three load-acquire in CPU0 synchronize with the three store-release
> in CPU2, but CPU0 and CPU1 are only synchronized up to gfn1 and CPU1
> may miss gfn2's fields other than flags.
>
> The kernel must be able to cope with invalid values of the fields, and
> userspace *will* invoke the ioctl once more. However, once the RESET flag
> is cleared on gfn2, it is lost forever, therefore in the above scenario
> CPU1 must read the correct value of gfn2's fields.
>
> Therefore RESET must be set with a store-release, that will synchronize
> with KVM's load-acquire in CPU1.
>
> Cc: Gavin Shan <gshan@redhat.com>
> Cc: Peter Xu <peterx@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Thanks (again)!
--
Peter Xu