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@xxxxxxxxxx > > Signed-off-by: Joerg Roedel<joerg.roedel@xxxxxxx> > > --- > > 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 -- AMD Operating System Research Center Advanced Micro Devices GmbH Einsteinring 24 85609 Dornach General Managers: Alberto Bozzo, Andrew Bowd Registration: Dornach, Landkr. Muenchen; Registerger. Muenchen, HRB Nr. 43632 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
