Re: [PATCH] tsc: use kvmclock for calibration

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Aug 09, 2012 at 03:53:24PM +0300, Avi Kivity wrote:
> On 08/09/2012 02:57 PM, Gerd Hoffmann wrote:
> > Use kvmclock for tsc calibration when running on kvm.  Without this the
> > tsc frequency calibrated by seabios can be *way* off in case the virtual
> > machine is booted on a loaded host.  I've seen seabios calibrating 27
> > instead of ca. 2800 MHz, resulting in timeouts being to short by factor
> > 100.  Which in turn leads to disk I/O errors due to timeouts, especially
> > as I/O requests tend to take a bit longer than usual on a loaded box ...
> 
> > +
> > +struct pvclock_vcpu_time_info {
> > +	u32   version;
> > +	u32   pad0;
> > +	u64   tsc_timestamp;
> > +	u64   system_time;
> > +	u32   tsc_to_system_mul;
> > +	s8    tsc_shift;
> > +	u8    flags;
> > +	u8    pad[2];
> > +} PACKED;
> > +
> > +
> > +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?  There is a 64-byte alignment requirement.
> 
> > +    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.

Thats why there exists hardware tsc frequency scaling and the software
equivalent for that on kvm.

> You want accurate time on kvm, don't use the tsc.
> 
> 
> -- 
> error compiling committee.c: too many arguments to function
> --
> To unsubscribe from this list: send the line "unsubscribe kvm" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html