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

Michael S. Tsirkin
2019-12-10T13:25:55Z

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