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

    --
    MST

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