Re: [PATCH 02/15] KVM: x86: Emulate RDPID only if RDTSCP is supported

[Maxim Levitsky]

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