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SubjectRe: [PATCH v2 1/1] KVM: x86: fix MSR_IA32_TSC read for nested migration
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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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