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

On 01/04/2016 07:59 PM, Haozhong Zhang wrote:
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.


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.

OK --- I was looking at tsc_set_info()'s TSC_MODE_NEVER_EMULATE path but now I realize that we don't use scaling in that mode (right?).

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>


  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 
-        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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