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

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Date:   Thu, 12 Dec 2019 02:36:13 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     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 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