Re: [PATCH] KVM: svm: add support for RDTSCP

Joerg Roedel
2015-11-12T14:45:09Z

Hi Paolo,

On Thu, Nov 12, 2015 at 02:49:16PM +0100, Paolo Bonzini wrote:
> RDTSCP was never supported for AMD CPUs, which nobody noticed because
> Linux does not use it. But exactly the fact that Linux does not
> use it makes the implementation very simple; we can freely trash
> MSR_TSC_AUX while running the guest.
>
> Cc: Joerg Roedel <joro@8bytes.org>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/kvm/svm.c | 24 +++++++++++++++++++++++-
> 1 file changed, 23 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 83a1c64..c302614 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -86,6 +86,7 @@ static const u32 host_save_user_msrs[] = {
> MSR_FS_BASE,
> #endif
> MSR_IA32_SYSENTER_CS, MSR_IA32_SYSENTER_ESP, MSR_IA32_SYSENTER_EIP,
> + MSR_TSC_AUX,
> };
>
> #define NR_HOST_SAVE_USER_MSRS ARRAY_SIZE(host_save_user_msrs)
> @@ -135,6 +136,7 @@ struct vcpu_svm {
> uint64_t asid_generation;
> uint64_t sysenter_esp;
> uint64_t sysenter_eip;
> + uint64_t tsc_aux;
>
> u64 next_rip;
>
> @@ -1238,6 +1240,9 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> wrmsrl(MSR_AMD64_TSC_RATIO, tsc_ratio);
> }
> }
> + /* This assumes that the kernel never uses MSR_TSC_AUX */
> + if (static_cpu_has(X86_FEATURE_RDTSCP))
> + wrmsrl(MSR_TSC_AUX, svm->tsc_aux);
> }

Hmm, you seem to still intercept MSR_TSC_AUX, is that intentional?
Loading the guests value into the real cpu msr only makes sense to me
when the MSR is no longer intercepted.

Joerg