Re: [Xen-devel] [PATCH v2 06/14] x86/time.c: Scale host TSC in pvclock properly

[Haozhong Zhang <haozhong.zhang@xxxxxxxxx>]

On 12/08/15 14:21, Boris Ostrovsky wrote:
> On 12/07/2015 08:52 PM, Haozhong Zhang wrote:
> >On 12/07/15 13:14, Boris Ostrovsky wrote:
> >>On 12/06/2015 03:58 PM, Haozhong Zhang wrote:
> >>>This patch makes the pvclock return the scaled host TSC and
> >>>corresponding scaling parameters to HVM domains if guest TSC is not
> >>>emulated and TSC scaling is enabled.
> >>>
> >>>Signed-off-by: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>
> >>+Joao who has been staring at this code.
> >>
> >>Joao, can you run this series through your test with non-native frequency?
> >>(http://lists.xenproject.org/archives/html/xen-devel/2015-12/msg00683.html
> >>provides an interface to set it in config file).
> >>
> >>
> >>>---
> >>>  xen/arch/x86/time.c | 16 ++++++++++++----
> >>>  1 file changed, 12 insertions(+), 4 deletions(-)
> >>>
> >>>diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
> >>>index 95df4f1..732d1e9 100644
> >>>--- a/xen/arch/x86/time.c
> >>>+++ b/xen/arch/x86/time.c
> >>>@@ -815,10 +815,18 @@ static void __update_vcpu_system_time(struct vcpu 
> >>>*v, int force)
> >>>      }
> >>>      else
> >>>      {
> >>>-        tsc_stamp = t->local_tsc_stamp;
> >>>-
> >>>-        _u.tsc_to_system_mul = t->tsc_scale.mul_frac;
> >>>-        _u.tsc_shift         = (s8)t->tsc_scale.shift;
> >>>+        if ( is_hvm_domain(d) && cpu_has_tsc_ratio )
> >>>+        {
> >>>+            tsc_stamp            = hvm_funcs.scale_tsc(v, 
> >>>t->local_tsc_stamp);
> >>>+            _u.tsc_to_system_mul = d->arch.vtsc_to_ns.mul_frac;
> >>>+            _u.tsc_shift         = d->arch.vtsc_to_ns.shift;
> >>I am not sure this is correct (which is why I asked Joao to look at this and
> >>test). The scaler below is calculated as result of TSC calibration across
> >>physical CPUs and what you use above (vtsc_to_ns) is an uncalibrated value.
> >>
> >Because guest TSC is synchronized among all vcpus of a domain, I think
> >it's safe to use d->arch.vtsc_to_ns here.
> 
> This is only guaranteed if we have constant/reliable TSC. So perhaps you
> should set tsc_scaling_supported only when either (or both?) of these TSC
> properties are true. Which is probably the case anyway but may be worth
> explicitly checking in start_svm/vmx?

Yes, I'll add the additional check in the next version.

> (and use tsc_scaling_supported instead
> of cpu_has_tsc_ratio in the 'if' statement)

This one is only for bug fix, so tsc_scaling_supported has not been
introduced. Patch 8 introduces tsc_scaling_supported and patch 12
replaces all cpu_has_tsc_ratio with it.

> 
> And just like I asked in the previous email --- should we then use the same
> scaler (which would be vtsc_to_ns) in both cases? At least for guests in HVM
> containers (it may work for PV guests as well, but it would need to be
> confirmed).
>
Yes, but I'll check PV code first.

> Also, I noticed that this routine uses is_hvm_domain(). I think it should be
> has_hvm_container_domain().
>
forgot updating here, will do in the next version.

Haozhong

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel