Re: [PATCH] KVM: SVM: fix random segfaults with NPT enabled

[Joerg Roedel <joro@8bytes.org>]

On Wed, Aug 27, 2008 at 06:21:40PM +0300, Avi Kivity wrote:
> Joerg Rodel wrote:
> 
> 
> 
> >>hm.  tbench doesn't allocate memory, so there shouldn't be any npt 
> >>faults.  I don't see how this can make a difference.
> >>    
> >
> >  
> 
> I reproduced it.  There are a few npt faults as the guest has not 
> touched all of memory yet.  If I force it to touch all of memory (dd < 
> /dev/hda), the problem appears to go away.
> 
> >Base for the fix was this bugreport:
> >
> >http://sourceforge.net/tracker/index.php?func=detail&aid=2019053&group_id=180599&atid=893831
> >
> >We found out that the same crash occur on long running compile
> >workloads and that stale tlb-entries cause it. Until we find the real
> >location of the missing tlb flush in the mmu code, i think its best to
> >flush the TLB every time the mapping/unmapping code for the nested page
> >table is executed. This fixes at least the crashes in the guest and has
> >only minimal performance impact.
> >  
> 
> I'd like to try and find out what the problem is exactly.  Otherwise we 
> may be only narrowing the window, not closing it.

Agreed. The fix I sent is only meant to be temporary until we find the
real root cause of the problem.

Joerg