Re: [PATCH 1/2] KVM: SVM: Fix NMI path when NMI happens in guest mode

["Roedel, Joerg" <Joerg.Roedel@amd.com>]

On Thu, Jan 13, 2011 at 10:42:01AM -0500, Avi Kivity wrote:
> On 01/13/2011 05:22 PM, Joerg Roedel wrote:
> > The vmexit path on SVM needs to restore the KERNEL_GS_BASE
> > MSR in order to savely execute the NMI handler. Otherwise a
> > pending NMI can occur after the STGI instruction and crash
> > the machine.
> > This makes it impossible to run perf and kvm in parallel on
> > an AMD machine in a stable way.
> >
> > Cc: stable@kernel.org
> > Signed-off-by: Joerg Roedel<joerg.roedel@amd.com>
> > ---
> >   arch/x86/kvm/svm.c |    1 +
> >   1 files changed, 1 insertions(+), 0 deletions(-)
> >
> > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > index 25bd1bc..8b9bc72 100644
> > --- a/arch/x86/kvm/svm.c
> > +++ b/arch/x86/kvm/svm.c
> > @@ -3637,6 +3637,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
> >
> >   #ifdef CONFIG_X86_64
> >   	wrmsrl(MSR_GS_BASE, svm->host.gs_base);
> > +	wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gs);
> >   #else
> >   	loadsegment(fs, svm->host.fs);
> >   #endif
> 
> Why would an NMI crash if MSR_KERNEL_GS_BASE is bad?
> 
> I see save_paranoid depends on MSR_GS_BASE (specifically its sign, which 
> is bad for the new instructions that allow userspace to write gsbase), 
> but not on MSR_KERNEL_GS_BASE.

Thats a good question. I have not idea. I spent some time trying to
figure this out (after I found out that wrong KERNEL_GS_BASE was the
cause of the crashes) but had no luck.

This also doesn't happen every time an NMI is delivered in svm_vcpu_run.
Sometimes it runs perfectly in parallel for a few minutues before the
machine triple-faults.

I also had a look at entry_64.S. The save_paranoid could not be the
cause because MSR_GS_BASE is already negative at this point. But the
re-schedule condition check at the end of the NMI handler code could
also not be the cause because the NMI happens while preemption (and
interrupts) are disabled (a re-schedule should also trigger
preempt-notifiers and restore KERNEL_GS_BASE).

	Joerg

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