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

From: Michael S. Tsirkin
Date: Thu Dec 12 2019 - 02:36:23 EST

On Thu, Dec 12, 2019 at 01:08:14AM +0100, Paolo Bonzini wrote:
> >> I'd say it won't be a big issue on locking 1/2M of host mem for a
> >> vm...
> >> Also note that if dirty ring is enabled, I plan to evaporate the
> >> dirty_bitmap in the next post. The old kvm->dirty_bitmap takes
> >> $GUEST_MEM/32K*2 mem. E.g., for 64G guest it's 64G/32K*2=4M. If with
> >> dirty ring of 8 vcpus, that could be 64K*8=0.5M, which could be even
> >> less memory used.
> >
> > Right - I think Avi described the bitmap in kernel memory as one of
> > design mistakes. Why repeat that with the new design?
> Do you have a source for that?

Nope, it was a private talk.

> At least the dirty bitmap has to be
> accessed from atomic context so it seems unlikely that it can be moved
> to user memory.

Why is that? We could surely do it from VCPU context?

> The dirty ring could use user memory indeed, but it would be much harder
> to set up (multiple ioctls for each ring? what to do if userspace
> forgets one? etc.).

Why multiple ioctls? If you do like virtio packed ring you just need the
base and the size.