linux-kernel - Re: [PATCH 4/4] kvm: Implement PEBS virtualization

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Date:	Wed, 25 Jun 2014 10:04:14 +0300
From:	Avi Kivity <avi.kivity@...il.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>,
	Andi Kleen <andi@...stfloor.org>
CC:	peterz@...radead.org, gleb@...nel.org, pbonzini@...hat.com,
	eranian@...gle.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 4/4] kvm: Implement PEBS virtualization


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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