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

From: Gleb Natapov
Date: Mon Jun 02 2014 - 12:45:45 EST

On Fri, May 30, 2014 at 09:24:24AM -0700, Andi Kleen wrote:
> > > To avoid any problems with guest pages being swapped by the host we
> > > pin the pages when the PEBS buffer is setup, by intercepting
> > > that MSR.
> > It will avoid guest page to be swapped, but shadow paging code may still drop
> > shadow PT pages that build a mapping from DS virtual address to the guest page.
> You're saying the EPT code could tear down the EPT mappings?

Under memory pressure yes. mmu_shrink_scan() calls
prepare_zap_oldest_mmu_page() which destroys oldest mmu pages like its
name says. As far as I can tell running nested guest can also result in
EPT mapping to be dropped since it will create a lot of shadow pages and
this will cause make_mmu_pages_available() to destroy some shadow pages
and it may choose EPT pages to destroy.

CCing Marcelo to confirm/correct.

> OK that would need to be prevented too. Any suggestions how?
Only high level. Mark shadow pages involved in translation we want to keep and skip them in

> > With EPT it is less likely to happen (but still possible IIRC depending on memory
> > pressure and how much memory shadow paging code is allowed to use), without EPT
> > it will happen for sure.
> Don't care about the non EPT case, this is white listed only for EPT supporting
> CPUs.
User may still disable EPT during module load, so pebs should be dropped
from a guest's cpuid in this case.

> > There is nothing, as far as I can see, that says what will happen if the
> > condition is not met. I always interpreted it as undefined behaviour so
> > anything can happen including CPU dies completely. You are saying above
> > on one hand that CPU cannot handle any kinds of faults during write to
> > DS area, but on the other hand a guest could only crash itself. Is this
> > architecturally guarantied?
> You essentially would get random page faults, and the PEBS event will
> be cancelled. No hangs.
Is this a guest who will get those random page faults or a host?

> It's not architecturally guaranteed, but we white list anyways so
> we only care about the white listed CPUs at this point. For them
> I have confirmation that it works.
> -Andi

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