Re: [RFC PATCH] x86: Remove X86_FEATURE_MFENCE_RDTSC

[Paolo Bonzini]

On 08/07/19 21:32, Lendacky, Thomas wrote:
>> AMD and Intel both have serializing lfence (X86_FEATURE_LFENCE_RDTSC).
>> They've both had it for a long time, and AMD has had it enabled in Linux
>> since Spectre v1 was announced.
>>
>> Back then, there was a proposal to remove the serializing mfence feature
>> bit (X86_FEATURE_MFENCE_RDTSC), since both AMD and Intel have
>> serializing lfence.  At the time, it was (ahem) speculated that some
>> hypervisors might not yet support its removal, so it remained for the
>> time being.
>>
>> Now a year-and-a-half later, it should be safe to remove.
>
> I vaguely remember a concern from a migration point of view, maybe? Adding
> Paolo to see if he has any concerns.

It would be a problem to remove the conditional "if
(boot_cpu_has(X86_FEATURE_LFENCE_RDTSC))" from svm_get_msr_feature.  But
removing support for X86_FEATURE_MFENCE_RDTSC essentially amounts to
removing support for hypervisors that haven't been updated pre-Spectre.
 That's fair enough, I think.

Paolo