Re: [PATCH v3 3/4] KVM: x86: Tie Intel and AMD behavior for MSR_TSC_AUX to guest CPU model

Reiji Watanabe
2021-04-27T04:43:12Z

> > It looks Table 2-2 of the Intel SDM Vol4 (April 2021) says
> > TSC_AUX is supported:
> >
> > If CPUID.80000001H:EDX[27] = 1 or CPUID.(EAX=7,ECX=0):ECX[22] = 1
> >
> > Should we also check X86_FEATURE_RDPID before returning 1
> > due to no RDTSCP support ?
>
> Yep. VMX should also clear RDPID if the ENABLE_RDTSCP control isn't supported.
> That bug isn't fatal because KVM emulates RDPID on #UD, but it would be a
> notieable performance hit for the guest.

Thank you so much for the confirmation and the information.
Understood.

> There is also a kernel bug lurking; vgetcpu_cpu_init() doesn't check
> X86_FEATURE_RDPID and will fail to initialize MSR_TSC_AUX if RDPID is supported
> but RDTSCP is not, and __getcpu() uses RDPID. I'll verify that's broken and
> send a patch for that one too.

I don't find vgetcpu_cpu_init() or __getcpu() in
https://github.com/torvalds/linux.
I would assume you meant setup_getcpu() and vdso_read_cpunode() instead (?).

> AMD also documents this in Appendix E:
>
> CPUID Fn0000_0007_EBX_x0 Structured Extended Feature Identifiers (ECX=0)
> Bits Field Name
> ...
> 22 RDPID RDPID instruction and TSC_AUX MSR support.

Thank you. I overlooked that...

Regards,
Reiji