Re: [tip:x86/pti] x86/cpu/AMD: Use LFENCE_RDTSC instead of MFENCE_RDTSC

From: Tom Lendacky
Date: Mon Jan 08 2018 - 12:39:42 EST


On 1/8/2018 11:01 AM, Paolo Bonzini wrote:
> On 08/01/2018 17:48, Dr. David Alan Gilbert wrote:
>>> If your hypervisor is lying to you about the primary family, then all
>>> bets are off. I don't expect there will be any production systems doing
>>> this.
>> It's not that an unusual thing to do on qemu/kvm - to specify the lowest
>> common denominator of the set of CPUs in your data centre (for any one
>> vendor); it does tend to get some weird combinations.
>
> Agreed. But on a hypervisor we pretty much know that:
>
> - the MSR_AMD64_DE_CFG doesn't exist unless you have a fix

Not sure what you mean by this... the MSR exists today on many families.

Thanks,
Tom

>
> - setting the MSR_AMD64_DE_CFG bit to 1 if you have a fix can be done
> independent of the family
>
> So all KVM needs is a X86_FEATURE_LFENCE_SERIALIZE, it doesn't matter if
> it's because of the family or because Linux has set MSR_F10H_DE_CFG.
> The guest will either try setting the MSR bit and #GP, or it will find
> it already set and do nothing.
>
> Of course no code for this has been written yet.
>
> Paolo
>