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

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Message-ID: <20191216154742.GF83861@xz-x1>
Date:   Mon, 16 Dec 2019 10:47:42 -0500
From:   Peter Xu <peterx@...hat.com>
To:     "Michael S. Tsirkin" <mst@...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 10:33:42AM -0500, Michael S. Tsirkin wrote:
> 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.

In our case even an old reset_index should not matter much here imho
because the worst case is we read an old reset so we stop pushing to a
ring when it's just being reset and at the same time it's soft-full
(so an extra user exit even race happened).  But I agree it's clearer
to READ_ONCE() on readers.  Thanks!

-- 
Peter Xu