Re: [Xen-devel] [PATCH 09/13] x86/time.c: Scale host TSC in pvclock properly

[Haozhong Zhang <haozhong.zhang@xxxxxxxxx>]

On Thu, Oct 22, 2015 at 11:50:18AM -0400, Boris Ostrovsky wrote:
> On 09/28/2015 03:13 AM, 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>
> >---
> >  xen/arch/x86/time.c | 15 ++++++++++++---
> >  1 file changed, 12 insertions(+), 3 deletions(-)
> >
> >diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
> >index 4b5402c..54eab6e 100644
> >--- a/xen/arch/x86/time.c
> >+++ b/xen/arch/x86/time.c
> >@@ -832,10 +832,19 @@ static void __update_vcpu_system_time(struct vcpu *v, 
> >int force)
> >      }
> >      else
> >      {
> >-        _u.tsc_timestamp     = t->local_tsc_stamp;
> >+        if ( is_hvm_domain(d) && hvm_funcs.tsc_scaling_supported )
> >+        {
> >+            _u.tsc_timestamp     = hvm_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;
> >+        }
> >+        else
> >+        {
> >+            _u.tsc_timestamp     = t->local_tsc_stamp;
> >+            _u.tsc_to_system_mul = t->tsc_scale.mul_frac;
> >+            _u.tsc_shift         = (s8)t->tsc_scale.shift;
> >+        }
> >          _u.system_time       = t->stime_local_stamp;
> >-        _u.tsc_to_system_mul = t->tsc_scale.mul_frac;
> >-        _u.tsc_shift         = (s8)t->tsc_scale.shift;
> >      }
> >      if ( is_hvm_domain(d) )
> >          _u.tsc_timestamp += v->arch.hvm_vcpu.cache_tsc_offset;
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
             the offset is subtract here
> 
> So this is not directly related to this series but when we calculate
> tsc_timestamp --- shouldn't we subtract TSC offset? Otherwise we are
> reporting (possibly scaled) host's TSC and this is supposed to be guest's
> counter.
> 
> 
> -boris

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