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

[Avi Kivity <avi@redhat.com>]

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.

-- 
error compiling committee.c: too many arguments to function