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

From: Paolo Bonzini
Date: Thu Dec 12 2019 - 03:12:13 EST

On 12/12/19 08:36, Michael S. Tsirkin wrote:
> 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?

Spinlock is taken.

>> 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.

You have multiple rings, so multiple invocations of one ioctl.