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

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Date:   Tue, 10 Dec 2019 08:25:43 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>, Peter Xu <peterx@...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 04, 2019 at 12:04:53PM +0100, Paolo Bonzini wrote:
> On 04/12/19 11:38, Jason Wang wrote:
> >>
> >> +    entry = &ring->dirty_gfns[ring->dirty_index & (ring->size - 1)];
> >> +    entry->slot = slot;
> >> +    entry->offset = offset;
> > 
> > 
> > Haven't gone through the whole series, sorry if it was a silly question
> > but I wonder things like this will suffer from similar issue on
> > virtually tagged archs as mentioned in [1].
> 
> 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?

If you did and it's not a good fit, this is something good to mention
in the commit log.

I also wonder about performance numbers - any data here?


> > Is this better to allocate the ring from userspace and set to KVM
> > instead? Then we can use copy_to/from_user() friends (a little bit slow
> > on recent CPUs).
> 
> Yeah, I don't think that would be better than mmap.
> 
> Paolo
> 
> 
> > [1] https://lkml.org/lkml/2019/4/9/5