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 <kra...@redhat.com>
> > ---
> > 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
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