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
