[SeaBIOS] Re: [PATCH 3/3] kvm: add support for reading tsc frequency via cpuid.

[Gerd Hoffmann]

On Fri, Mar 06, 2020 at 02:14:01PM -0500, Kevin O'Connor wrote:
> On Fri, Mar 06, 2020 at 04:44:16PM +0100, Gerd Hoffmann wrote:
> > Signed-off-by: Gerd Hoffmann
> >
> > src/fw/paravirt.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git a/src/fw/paravirt.c b/src/fw/paravirt.c
> > index 4fcd8f570673..8b463af96c3e 100644
> > --- a/src/fw/paravirt.c
> > +++ b/src/fw/paravirt.c
> > @@ -67,6 +67,11 @@ static void kvm_detect(void)
> >      if (strcmp(signature, "KVMKVMKVM") == 0) {
> >          dprintf(1, "Running on KVM\n");
> >          PlatformRunningOn |= PF_KVM;
> >    if (eax >= KVM_CPUID_SIGNATURE + 0x10) {
> >        cpuid(KVM_CPUID_SIGNATURE + 0x10, &eax, &ebx, &ecx, &edx);
> >        dprintf(1, "kvm: have invtsc, freq %u kHz\n", eax);
> >        tsctimer_setfreq(eax);
> >    }
> > }
> >
> > }
>
> That's odd that a khz value is being passed as eax.  Is this a recent
> change?

KVM paravirtual extension.  Exists for years.  I think it's off by
default (didn't check all cpu models though).  You'll get it with "qemu
-cpu $model,invtsc=on".

When available it is possible to use the tsc as reliable clock source
because you don't need calibration which can be way off on a loaded
host.

I'm looking for a clock source which works in case pmtimer is not
available.  Maybe I should just go for kvmclock instead ...

cheers,
  Gerd