Re: [Xen-devel] [PATCH v3 04/13] x86/time.c: Scale host TSC in pvclock properly

[Haozhong Zhang <haozhong.zhang@xxxxxxxxx>]

On 01/04/16 13:09, Boris Ostrovsky wrote:
> On 12/30/2015 10:03 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>
> >---
> >Changes in v3:
> >  (addressing Boris Ostrovsky's comments)
> >  * No changes in fact. tsc_set_info() does not set d->arch.vtsc to 0
> >    if host_tsc_is_safe() is not satisfied, so it is safe to use
> >    d->arch.vtsc_to_ns here.
> 
> I don't think I understand your argument here. I think last time we decided
> that vtsc_to_ns can be used only if TSC is constant.
> 
> -boris
>
In tsc_set_info(), if TSC scaling is available and host has
X86_FEATURE_TSC_RELIABLE (checked by host_tsc_is_safe()), vtsc is left
to 0. And looking at init_amd() and init_intel(),
X86_FEATURE_CONSTANT_TSC is available whenever
X86_FEATURE_TSC_RELIABLE is available as well. Therefore, when
vtsc_to_ns is used in __update_vcpu_system_time(), we can always
ensure that host has X86_FEATURE_CONSTANT_TSC and do not need to
recheck it there.

Haozhong

> >
> >  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 d83f068..b31634a 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 ( has_hvm_container_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;
> >+        }
> >+        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;
> >+        }
> >      }
> >      _u.tsc_timestamp = tsc_stamp;
> 

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