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To:     Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>
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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 8/23/22 22:35, Marc Zyngier wrote:
>> Heh, yeah I need to get that out the door. I'll also note that Gavin's
>> changes are still relevant without that series, as we do write unprotect
>> in parallel at PTE granularity after commit f783ef1c0e82 ("KVM: arm64:
>> Add fast path to handle permission relaxation during dirty logging").
>
> Ah, true. Now if only someone could explain how the whole
> producer-consumer thing works without a trace of a barrier, that'd be
> great...

Do you mean this?

void kvm_dirty_ring_push(struct kvm_dirty_ring *ring, u32 slot, u64 offset)
{
         struct kvm_dirty_gfn *entry;

         /* It should never get full */
         WARN_ON_ONCE(kvm_dirty_ring_full(ring));

         entry = &ring->dirty_gfns[ring->dirty_index & (ring->size - 1)];

         entry->slot = slot;
         entry->offset = offset;
         /*
          * Make sure the data is filled in before we publish this to
          * the userspace program.  There's no paired kernel-side reader.
          */
         smp_wmb();
         kvm_dirty_gfn_set_dirtied(entry);
         ring->dirty_index++;
         trace_kvm_dirty_ring_push(ring, slot, offset);
}

The matching smp_rmb() is in userspace.

Paolo