On Tue, 2020-09-22 at 14:50 +0200, Paolo Bonzini wrote: > On 21/09/20 18:23, Sean Christopherson wrote: > > Avoid "should" in code comments and describe what the code is doing, not what > > it should be doing. The only exception for this is when the code has a known > > flaw/gap, e.g. "KVM should do X, but because of Y, KVM actually does Z". > > > > > + * return it's real L1 value so that its restore will be correct. > > s/it's/its > > > > Perhaps add "unconditionally" somewhere, since arch.tsc_offset can also contain > > the L1 value. E.g. > > > > * Unconditionally return L1's TSC offset on userspace reads > > * so that userspace reads and writes always operate on L1's > > * offset, e.g. to ensure deterministic behavior for migration. > > */ > > > > Technically the host need not restore MSR_IA32_TSC at all. This follows > the idea of the discussion with Oliver Upton about transmitting the > state of the kvmclock heuristics to userspace, which include a (TSC, > CLOCK_MONOTONIC) pair to transmit the offset to the destination. All > that needs to be an L1 value is then the TSC value in that pair. > > I'm a bit torn over this patch. On one hand it's an easy solution, on > the other hand it's... just wrong if KVM_GET_MSR is used for e.g. > debugging the guest. Could you explain why though? After my patch, the KVM_GET_MSR will consistently read the L1 TSC, just like all other MSRs as I explained. I guess for debugging, this should work? The fact that TSC reads with the guest offset is a nice exception made for the guests, that insist on reading this msr without inteception and not using rdtsc. Best regards, Maxim Levitsky > > I'll talk to Maxim and see if he can work on the kvmclock migration stuff. > > Paolo >
