linux-kernel - Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

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Message-ID: <20191216103251-mutt-send-email-mst@kernel.org>
Date:   Mon, 16 Dec 2019 10:33:42 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

On Mon, Dec 16, 2019 at 10:07:54AM -0500, Peter Xu wrote:
> On Mon, Dec 16, 2019 at 04:47:36AM -0500, Michael S. Tsirkin wrote:
> > On Sun, Dec 15, 2019 at 12:33:02PM -0500, Peter Xu wrote:
> > > On Thu, Dec 12, 2019 at 01:08:14AM +0100, Paolo Bonzini wrote:
> > > > >>> What depends on what here? Looks suspicious ...
> > > > >>
> > > > >> Hmm, I think maybe it can be removed because the entry pointer
> > > > >> reference below should be an ordering constraint already?
> > > > 
> > > > entry->xxx depends on ring->reset_index.
> > > 
> > > Yes that's true, but...
> > > 
> > >         entry = &ring->dirty_gfns[ring->reset_index & (ring->size - 1)];
> > >         /* barrier? */
> > >         next_slot = READ_ONCE(entry->slot);
> > >         next_offset = READ_ONCE(entry->offset);
> > > 
> > > ... I think entry->xxx depends on entry first, then entry depends on
> > > reset_index.  So it seems fine because all things have a dependency?
> > 
> > Is reset_index changed from another thread then?
> > If yes then you want to read reset_index with READ_ONCE.
> > That includes a dependency barrier.
> 
> There're a few readers, but only this function will change it
> (kvm_dirty_ring_reset).  Thanks,

Then you don't need any barriers in this function.
readers need at least READ_ONCE.

> -- 
> Peter Xu