On 12/11/24 16:22, Borislav Petkov wrote: > On Wed, Dec 11, 2024 at 04:01:31PM -0600, Tom Lendacky wrote: >> It could be any reason... maybe the hypervisor wants to know when this >> MSR used in order to tell the guest owner to update their code. Writing >> to or reading from that MSR is not that common, so I would think we want >> to keep the same behavior that has been in effect. > > Ah, I thought you're gonna say something along the lines of, yeah, we must use > the HV GHCB protocol because of <raisins> and there's no other way this could > work. > >> But if we do want to make this change, maybe do it separate from the >> Secure TSC series since it alters the behavior of SEV-ES guests and >> SEV-SNP guests without Secure TSC. > > Already suggested so - this should be a separate patch. > > It would be interesting to see if it brings any improvement by avoiding the HV > call... especially since RDTSC is used a *lot* and prominently at that in > sched_clock, for example. I doubt you would notice anything since it doesn't look like Linux ever reads from or writes to MSR_IA32_TSC, preferring to just use RDTSC or RDTSCP which aren't usually intercepted at all and so never goes out to the HV. Thanks, Tom >
