Re: [PATCH v2 4/4] KVM: nVMX: Add support for capturing highest observable L2 TSC

[Jim Mattson <jmattson@google.com>]

On Tue, Nov 5, 2019 at 1:58 PM Liran Alon <liran.alon@oracle.com> wrote:
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>
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> > On 5 Nov 2019, at 21:19, Aaron Lewis <aaronlewis@google.com> wrote:
> >
> > The L1 hypervisor may include the IA32_TIME_STAMP_COUNTER MSR in the
> > vmcs12 MSR VM-exit MSR-store area as a way of determining the highest
> > TSC value that might have been observed by L2 prior to VM-exit. The
> > current implementation does not capture a very tight bound on this
> > value.  To tighten the bound, add the IA32_TIME_STAMP_COUNTER MSR to the
> > vmcs02 VM-exit MSR-store area whenever it appears in the vmcs12 VM-exit
> > MSR-store area.  When L0 processes the vmcs12 VM-exit MSR-store area
> > during the emulation of an L2->L1 VM-exit, special-case the
> > IA32_TIME_STAMP_COUNTER MSR, using the value stored in the vmcs02
> > VM-exit MSR-store area to derive the value to be stored in the vmcs12
> > VM-exit MSR-store area.
> >
> > Reviewed-by: Jim Mattson <jmattson@google.com>
> > Signed-off-by: Aaron Lewis <aaronlewis@google.com>
>
> The patch looks correct to me and I had only some minor style comments below.
> Reviewed-by: Liran Alon <liran.alon@oracle.com>
>
> I think you may also consider to separate this patch into two:
> First patch add all framework code without still using it specifically for MSR_IA32_TSC
> and a second patch to use the framework for MSR_IA32_TSC case.
>
> Just out of curiosity, may I ask which L1 hypervisor use this technique that you encountered this issue?

It's a proprietary type 2 hypervisor that runs on Linux.

> -Liran