Re: [PATCH v1 1/5] KVM: arm64: Enable ring-based dirty memory tracking

[Paolo Bonzini]

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