Re: [PATCH] x86/hyperv: add comment describing TSC_INVARIANT_CONTROL MSR setting bit 0

[Ani Sinha]

On Fri, 16 Jul 2021, Vitaly Kuznetsov wrote:

> Ani Sinha <ani@xxxxxxxxxxx> writes:
>
> > Commit dce7cd62754b5 ("x86/hyperv: Allow guests to enable InvariantTSC")
> > added the support for HV_X64_MSR_TSC_INVARIANT_CONTROL. Setting bit 0
> > of this synthetic MSR will allow hyper-v guests to report invariant TSC
> > CPU feature through CPUID. This comment adds this explanation to the code
> > and mentions where the Intel's generic platform init code reads this
> > feature bit from CPUID. The comment will help developers understand how
> > the two parts of the initialization (hyperV specific and non-hyperV
> > specific generic hw init) are related.
> >
> > Signed-off-by: Ani Sinha <ani@xxxxxxxxxxx>
> > ---
> >  arch/x86/kernel/cpu/mshyperv.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> > index 715458b7729a..d2429748170d 100644
> > --- a/arch/x86/kernel/cpu/mshyperv.c
> > +++ b/arch/x86/kernel/cpu/mshyperv.c
> > @@ -368,6 +368,14 @@ static void __init ms_hyperv_init_platform(void)
> >  	machine_ops.crash_shutdown = hv_machine_crash_shutdown;
> >  #endif
> >  	if (ms_hyperv.features & HV_ACCESS_TSC_INVARIANT) {
> > +		/*
> > +		 * Setting bit 0 of the synthetic MSR 0x40000118 enables
> > +		 * guests to report invariant TSC feature through CPUID
> > +		 * instruction, CPUID 0x800000007/EDX, bit 8. See code in
> > +		 * early_init_intel() where this bit is examined. The
> > +		 * setting of this MSR bit should happen before init_intel()
> > +		 * is called.
>
> It should probably be emphasized, that write to 0x40000118
> updates/changes guest visible CPUIDs. This may not be clear as CPUIDs
> are generally considered 'static'.

Patch v2 sent.