On Tue, 2021-05-04 at 10:17 -0700, Sean Christopherson wrote:
> Do not advertise emulation support for RDPID if RDTSCP is unsupported.
> RDPID emulation subtly relies on MSR_TSC_AUX to exist in hardware, as
> both vmx_get_msr() and svm_get_msr() will return an error if the MSR is
> unsupported, i.e. ctxt->ops->get_msr() will fail and the emulator will
> inject a #UD.
>
> Note, RDPID emulation also relies on RDTSCP being enabled in the guest,
> but this is a KVM bug and will eventually be fixed.
>
> Fixes: fb6d4d340e05 ("KVM: x86: emulate RDPID")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
> arch/x86/kvm/cpuid.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index f765bf7a529c..c96f79c9fff2 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -637,7 +637,8 @@ static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
> case 7:
> entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> entry->eax = 0;
> - entry->ecx = F(RDPID);
> + if (kvm_cpu_cap_has(X86_FEATURE_RDTSCP))
> + entry->ecx = F(RDPID);
> ++array->nent;
> default:
> break;
Just to make sure that I understand this correctly:
This is what I know:
Both RDTSCP and RDPID are instructions that read IA32_TSC_AUX
(and RDTSCP also reads the TSC).
Both instructions have their own CPUID bits (X86_FEATURE_RDPID, X86_FEATURE_RDTSCP)
If either of these CPUID bits are present, IA32_TSC_AUX should be supported.
RDPID is a newer feature, thus I can at least for the sanity sake assume that
usually a CPU will either have neither of the features, have only RDTSCP,
and IA32_AUX, or have both RDSCP and RDPID.
If not supported in hardware KVM only emulates RDPID as I see.
Why btw? Performance wise guest that only wants the IA32_AUX in userspace,
is better to use RDTSCP and pay the penalty of saving/restoring of the
unwanted registers, than use RDPID with a vmexit.
My own guess for an answer to this question is that RDPID emulation is there
to aid migration from a host that does support RDPID to a host that doesn't.
Having said all that, assuming that we don't want to emulate the RDTSCP too,
when it is not supported, then this patch does make sense.
Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
Best regards,
Maxim Levitsky
