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

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

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).

Also, I noticed that this routine uses is_hvm_domain(). I think it should be has_hvm_container_domain().


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