Re: [PATCH 4/4] kvm: Implement PEBS virtualization

[Avi Kivity]

On 06/24/2014 07:45 PM, Marcelo Tosatti wrote:
> On Sun, Jun 22, 2014 at 09:02:25PM +0200, Andi Kleen wrote:
> > > First, it's not sufficient to pin the debug store area, you also
> > > have to pin the guest page tables that are used to map the debug
> > > store.  But even if you do that, as soon as the guest fork()s, it
> > > will create a new pgd which the host will be free to swap out.  The
> > > processor can then attempt a PEBS store to an unmapped address which
> > > will fail, even though the guest is configured correctly.
> > That's a good point. You're right of course.
> >
> > The only way I can think around it would be to intercept CR3 writes
> > while PEBS is active and always pin all the table pages leading
> > to the PEBS buffer. That's slow, but should be only needed
> > while PEBS is running.
> >
> > -Andi
> Suppose that can be done separately from the pinned spte patchset.
> And it requires accounting into mlock limits as well, as noted.
>
> One set of pagetables per pinned virtual address leading down to the
> last translations is sufficient per-vcpu.

Or 4, and use the CR3 exit filter to prevent vmexits among the last 4 
LRU CR3 values.
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