On 17/08/20 21:40, Oliver Upton wrote: >> If you don't have time to work on it I can try to find some for 5.10, >> but I'm not sure exactly when. > > Shouldn't be an issue, I'll futz around with some changes to the > series and send them out in the coming weeks. Ok, after looking more at the code with Maxim I can confidently say that it's a total mess. And a lot of the synchronization code is dead because 1) as far as we could see no guest synchronizes the TSC using MSR_IA32_TSC; and 2) writing to MSR_IA32_TSC_ADJUST does not trigger the synchronization code in kvm_write_tsc. Your patch works not by some sort of miracle, but rather because it bypasses the mess and that's the smart thing to do. The plan is now as follows: 1) guest-initiated MSR_IA32_TSC write never goes through the sync heuristics. I'll shortly send a patch for this, and it will fix the testcase issue 2) to have a new KVM_X86_DISABLE_QUIRKS value, that will toggle between "magic" and "vanilla" semantics for host-initiated TSC and TSC_ADJUST writes 3) if the quirk is present we still want existing userspace to work so: - host-initiated MSR_IA32_TSC write always returns the L1 TSC as in Maxim's recent patch. They will also always go through the sync heuristics. - host-initiated MSR_IA32_TSC_ADJUST write don't make the TSC jump, they only write to vcpu->arch.ia32_tsc_adjust_msr (as in the current kernel) 4) if the quirk is disabled however: - the sync heuristics are never used except in kvm_arch_vcpu_postcreate - host-initiated MSR_IA32_TSC and MSR_IA32_TSC_ADJUST accesses work like in the guest: reads of MSR_IA32_TSC return the "right" TSC, writes of MSR_IA32_TSC_ADJUST writes make the TSC jump. - for live migration, userspace is expected to use the new KVM_GET/SET_TSC_PRECISE (or whatever the name will be) to get/set a (nanosecond, TSC, TSC_ADJUST) tuple. The sync heuristics will be bypassed and it will just set the right value for the MSRs. Setting MSR_IA32_TSC_ADJUST is optional and controlled by a flag in the struct, and the flag will be set by KVM_GET_TSC_PRECISE based on the guest CPUID. Paolo
