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Re: [Xen-devel] [PATCH v2 04/14] x86/time.c: Use correct guest TSC frequency in tsc_get_info()

On 12/07/2015 08:08 PM, Haozhong Zhang wrote:
On 12/07/15 12:53, Boris Ostrovsky wrote:
On 12/06/2015 03:58 PM, Haozhong Zhang wrote:
When the TSC mode of a HVM container is TSC_MODE_DEFAULT or
TSC_MODE_PVRDTSCP and no TSC emulation is used, the existing
tsc_get_info() uses the host TSC frequency (cpu_khz) as the guest TSC
frequency. However, tsc_set_info() may set the guest TSC frequency to a
value different than the host. In order to keep consistent to
tsc_set_info(), this patch makes tsc_get_info() use the value set by
tsc_set_info() as the guest TSC frequency.

Signed-off-by: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>
  xen/arch/x86/time.c | 12 +++++++++---
  1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index 1091e69..95df4f1 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1749,6 +1749,9 @@ void tsc_get_info(struct domain *d, uint32_t *tsc_mode,
                    uint64_t *elapsed_nsec, uint32_t *gtsc_khz,
                    uint32_t *incarnation)
+    bool_t enable_tsc_scaling = has_hvm_container_domain(d) &&
+                                cpu_has_tsc_ratio;
&& !d->arch.vtsc ?

(assuming my comment to the previous patch is correct).

enable_tsc_scaling in tsc_get_info() is always used in the condition
!d->arch.vtsc, so it's not necessary to include it again in

      *incarnation = d->arch.incarnation;
      *tsc_mode = d->arch.tsc_mode;
@@ -1769,7 +1772,7 @@ void tsc_get_info(struct domain *d, uint32_t *tsc_mode,
          tsc = rdtsc();
          *elapsed_nsec = scale_delta(tsc, &d->arch.vtsc_to_ns);
-        *gtsc_khz = cpu_khz;
+        *gtsc_khz = enable_tsc_scaling ? d->arch.tsc_khz : cpu_khz;
          if ( d->arch.vtsc )
@@ -1779,10 +1782,13 @@ void tsc_get_info(struct domain *d, uint32_t *tsc_mode,
+            struct time_scale *scale = enable_tsc_scaling ?
+                                       &this_cpu(cpu_time).tsc_scale :
+                                       &d->arch.vtsc_to_ns;
IIUIC tsc_scale is host property and so why would it be used if TSC scaling
is available to guests?

scale is used below to convert a host TSC to nanosec. When TSC scaling is used,
d->arch.vtsc_to_ns may not base on the host TSC frequency, so I turn to the host
tsc_scale instead.

OK. Can we then use tsc_scale for both with and without TSC scaling? (on the set side too).

(And as a side note, I think that the comment in include/asm-x86/domain.h describing vtsc_to_ns as being used for emulated cases is rather misleading. We use it for both emulated and native)




              tsc = rdtsc();
-            *elapsed_nsec = scale_delta(tsc, &d->arch.vtsc_to_ns) -
+            *elapsed_nsec = scale_delta(tsc, scale) -
-            *gtsc_khz = 0; /* ignored by tsc_set_info */
+            *gtsc_khz = enable_tsc_scaling ? d->arch.tsc_khz : 0;

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