Messages in this thread | | | Subject | Re: [PATCH v2 1/1] KVM: x86: fix MSR_IA32_TSC read for nested migration | From | Paolo Bonzini <> | Date | Tue, 22 Sep 2020 14:50:34 +0200 |
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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.
I'll talk to Maxim and see if he can work on the kvmclock migration stuff.
Paolo
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