Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: [PATCH] KVM: svm: add support for RDTSCP | From | Paolo Bonzini <> | Date | Thu, 12 Nov 2015 17:18:07 +0100 |
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On 12/11/2015 15:45, Joerg Roedel wrote: > 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?
Yes. If I didn't intercept MSR_TSC_AUX, I would have to read it into svm->tsc_aux on every svm_vcpu_put. Because writing MSR_TSC_AUX is a rare operation, I intercept the write, and update both svm->tsc_aux and the processor MSR_TSC_AUX in svm_set_msr.
This is different from other host_save_msrs because the processor does not save MSR_TSC_AUX automatically in the VMCB. As remarked above, it only works because the kernel never uses RDTSCP.
> Loading the guests value into the real cpu msr only makes sense to me > when the MSR is no longer intercepted.
It's necessary for the correct operation of RDTSCP. Since we don't intercept RDTSCP, we need to load MSR_TSC_AUX.
Paolo
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