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



On 10/22/2015 12:44 PM, Haozhong Zhang wrote:
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

Ah, I missed this. Thanks.

-boris


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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