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

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Date:   Wed, 11 Dec 2019 09:54:04 -0500
From:   Peter Xu <peterx@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jason Wang <jasowang@...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 Wed, Dec 11, 2019 at 08:04:36AM -0500, Michael S. Tsirkin wrote:
> On Wed, Dec 11, 2019 at 10:05:28AM +0100, Paolo Bonzini wrote:
> > On 10/12/19 22:53, Michael S. Tsirkin wrote:
> > > On Tue, Dec 10, 2019 at 11:02:11AM -0500, Peter Xu wrote:
> > >> On Tue, Dec 10, 2019 at 02:31:54PM +0100, Paolo Bonzini wrote:
> > >>> On 10/12/19 14:25, Michael S. Tsirkin wrote:
> > >>>>> There is no new infrastructure to track the dirty pages---it's just a
> > >>>>> different way to pass them to userspace.
> > >>>> Did you guys consider using one of the virtio ring formats?
> > >>>> Maybe reusing vhost code?
> > >>>
> > >>> There are no used/available entries here, it's unidirectional
> > >>> (kernel->user).
> > >>
> > >> Agreed.  Vring could be an overkill IMHO (the whole dirty_ring.c is
> > >> 100+ LOC only).
> > > 
> > > I guess you don't do polling/ event suppression and other tricks that
> > > virtio came up with for speed then?
> 
> I looked at the code finally, there's actually available, and fetched is
> exactly like used. Not saying existing code is a great fit for you as
> you have an extra slot parameter to pass and it's reversed as compared
> to vhost, with kernel being the driver and userspace the device (even
> though vringh might fit, yet needs to be updated to support packed rings
> though).  But sticking to an existing format is a good idea IMHO,
> or if not I think it's not a bad idea to add some justification.

Right, I'll add a small paragraph in the next cover letter to justify.

Thanks,

-- 
Peter Xu