Re: [PATCH] tsc: use kvmclock for calibration

[Avi Kivity <avi@redhat.com>]

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.

-- 
error compiling committee.c: too many arguments to function