Re: [PATCH] KVM: use store-release to mark dirty pages as harvested

[Peter Xu <peterx@redhat.com>]

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