lkml.org 
[lkml]   [2014]   [Jun]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 4/4] kvm: Implement PEBS virtualization
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
prepare_zap_oldest_mmu_page().

>
> > 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

--
Gleb.


\
 
 \ /
  Last update: 2014-06-02 19:21    [W:0.143 / U:0.084 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site