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

[Maxim Levitsky]

On Mon, 2021-05-10 at 17:20 +0000, Sean Christopherson wrote:
> On Mon, May 10, 2021, Maxim Levitsky wrote:
> > 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.
> 
> Yep.
> 
> > 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.
> 
> Yep.
> 
> > If not supported in hardware KVM only emulates RDPID as I see.
> 
> Yep.
> 
> > 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.
> 
> FWIW, Linux doesn't even fall back to RDTSCP.  If RDPID isn't supported, Linux
> throws the info into the limit of a dummy segment in the GDT and uses LSL to get
> at the data.  Turns out that RDTSCP is too slow for its intended use case :-)
> 
> > 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.
> 
> That's my assumption as well.  Paolo's commit is a bit light on why emulation
> was added in the first place, but emulating to allow migrating to old hardware
> is the only motivation I can come up with.

Cool thanks!
Best regards,
	Maxim Levitsky

> 
> commit fb6d4d340e0532032c808a9933eaaa7b8de435ab
> Author: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Date:   Tue Jul 12 11:04:26 2016 +0200
> 
>     KVM: x86: emulate RDPID
> 
>     This is encoded as F3 0F C7 /7 with a register argument.  The register
>     argument is the second array in the group9 GroupDual, while F3 is the
>     fourth element of a Prefix.
> 
> > 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
> > 
> >