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To:     Marc Zyngier <maz@kernel.org>, Peter Xu <peterx@redhat.com>
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From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v1 1/5] KVM: arm64: Enable ring-based dirty memory
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On 8/24/22 00:47, Marc Zyngier wrote:
>> I definitely don't think I 100% understand all the ordering things since
>> they're complicated.. but my understanding is that the reset procedure
>> didn't need memory barrier (unlike pushing, where we have explicit wmb),
>> because we assumed the userapp is not hostile so logically it should only
>> modify the flags which is a 32bit field, assuming atomicity guaranteed.
> Atomicity doesn't guarantee ordering, unfortunately. Take the
> following example: CPU0 is changing a bunch of flags for GFNs A, B, C,
> D that exist in the ring in that order, and CPU1 performs an ioctl to
> reset the page state.
> 
> CPU0:
>      write_flag(A, KVM_DIRTY_GFN_F_RESET)
>      write_flag(B, KVM_DIRTY_GFN_F_RESET)
>      write_flag(C, KVM_DIRTY_GFN_F_RESET)
>      write_flag(D, KVM_DIRTY_GFN_F_RESET)
>      [...]
> 
> CPU1:
>     ioctl(KVM_RESET_DIRTY_RINGS)
> 
> Since CPU0 writes do not have any ordering, CPU1 can observe the
> writes in a sequence that have nothing to do with program order, and
> could for example observe that GFN A and D have been reset, but not B
> and C. This in turn breaks the logic in the reset code (B, C, and D
> don't get reset), despite userspace having followed the spec to the
> letter. If each was a store-release (which is the case on x86), it
> wouldn't be a problem, but nothing calls it in the documentation.
> 
> Maybe that's not a big deal if it is expected that each CPU will issue
> a KVM_RESET_DIRTY_RINGS itself, ensuring that it observe its own
> writes. But expecting this to work across CPUs without any barrier is
> wishful thinking.

Agreed, but that's a problem for userspace to solve.  If userspace wants 
to reset the fields in different CPUs, it has to synchronize with its 
own invoking of the ioctl.

That is, CPU0 must ensure that a ioctl(KVM_RESET_DIRTY_RINGS) is done 
after (in the memory-ordering sense) its last write_flag(D, 
KVM_DIRTY_GFN_F_RESET).  If there's no such ordering, there's no 
guarantee that the write_flag will have any effect.

The main reason why I preferred a global KVM_RESET_DIRTY_RINGS ioctl was 
because it takes kvm->slots_lock so the execution would be serialized 
anyway.  Turning slots_lock into an rwsem would be even worse because it 
also takes kvm->mmu_lock (since slots_lock is a mutex, at least two 
concurrent invocations won't clash with each other on the mmu_lock).

Paolo