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

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Date:   Thu, 12 Dec 2019 05:38:02 -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 09:12:04AM +0100, Paolo Bonzini wrote:
> 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.

Right, that's an implementation detail though isn't it?

> >> 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.
> 
> Paolo

Oh. So when you said "multiple ioctls for each ring" - I guess you
meant: "multiple ioctls - one for each ring"?

And it's true, but then it allows supporting things like resize in a
clean way without any effort in the kernel. You get a new ring address -
you switch to that one.

-- 
MST