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

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Message-ID: <20191211075413-mutt-send-email-mst@kernel.org>
Date:   Wed, 11 Dec 2019 08:04:36 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Peter Xu <peterx@...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 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.

> There are no interrupts either, so no need for event suppression.  You
> have vmexits when the ring gets full (and that needs to be synchronous),
> but apart from that the migration thread will poll the rings once when
> it needs to send more pages.
> 
> Paolo

OK don't use that then.

-- 
MST