On Thu, Aug 09, 2012 at 05:01:34PM +0300, Avi Kivity wrote:
> On 08/09/2012 04:57 PM, Gerd Hoffmann wrote:
> > Hi,
> >
> >>> +u64 kvm_tsc_khz(void)
> >>> +{
> >>> + u32 eax, ebx, ecx, edx, msr;
> >>> + struct pvclock_vcpu_time_info time;
> >>> + u32 addr = (u32)(&time);
> >>> + u64 khz;
> >>> +
> >>> + /* check presence and figure msr number */
> >>> + cpuid(KVM_CPUID_FEATURES, &eax, &ebx, &ecx, &edx);
> >>> + if (eax & KVM_FEATURE_CLOCKSOURCE2) {
> >>> + msr = MSR_KVM_SYSTEM_TIME_NEW;
> >>> + } else if (eax & KVM_FEATURE_CLOCKSOURCE) {
> >>> + msr = MSR_KVM_SYSTEM_TIME;
> >>> + } else {
> >>> + return 0;
> >>> + }
> >>> +
> >>> + /* ask kvm hypervisor to fill struct */
> >>> + memset(&time, 0, sizeof(time));
> >>> + wrmsr(msr, addr | 1);
> >>
> >> How can this work?
> >
> > It did in my testing, although maybe by pure luck ...
> >
> >> There is a 64-byte alignment requirement.
> >
> > 64 bytes? Sure? The whole struct is only 32 bytes in size ...
>
> er, the documentation says 4 bytes (so stack alignment works). I
> distinctly remember having a large alignment requirement so we don't
> cross a page or slot boundary... something's wrong here.
>
> >
> > Easily fixable though, just need to grab some memory with memalign
> > instead of using the stack.
>
> >
> >>> + wrmsr(msr, 0);
> >>> + if (time.version < 2 || time.tsc_to_system_mul == 0)
> >>> + return 0;
> >>> +
> >>> + /* go figure tsc frequency */
> >>> + khz = pvclock_tsc_khz(&time);
> >>> + dprintf(1, "Using kvmclock, msr 0x%x, tsc %d MHz\n",
> >>> + msr, (u32)khz / 1000);
> >>> + return khz;
> >>
> >> That's a meaningless number. You can be migrated to a cpu or a machine
> >> with very different tsc.
> >
> >> You want accurate time on kvm, don't use the tsc.
> >
> > seabios uses the tsc for timeout calculations only, so it doesn't need
> > to be 100% accurate. The order of magnitude should be correct though.
> > The Linux kernel uses the value for delay loops too, so using it for the
> > given purpose can't be *that* horrible after all ...
> >
> > It is certainly an improvement over the current code which tries to
> > calibrate the tsc and gets totally broken results in case the busy host
> > happens to schedule the guest in the middle of calibration.
> >
> > So what do you suggest? The options I see are:
> >
> > (1) Use this patch (with alignment issue fixed of course).
> > (2) Do a full kvmclock implementation. Feels a bit like overkill.
> > (3) SeaBIOS can fallback to the PIT for timing on machines which
> > have no TSC. We could do that too in case we detect kvm ...
>
> What sort of timeouts are these? If seconds, maybe the rtc would be best.
I vote for 3 so nobody has to maintain kvmclock code in SeaBIOS and Gerd
can fix the in-kernel PIT issues with GRUB (see Michaels message) while testing.
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