Re: [Xen-devel] [PATCH v3 02/13] x86/time.c: Use correct guest TSC frequency in tsc_get_info()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.01.16 at 18:48, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 12/30/2015 10:03 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>
>> ---
>> Changes in v3:
>>   (addressing Boris Ostrovsky's comments)
>>   * Use this_cpu(cpu_time).tsc_scale for both scaling and non-scaling cases.
>>
>>   xen/arch/x86/time.c | 9 ++++++---
>>   1 file changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
>> index 0059b6a..d83f068 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;
>> +
> 
> Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> 
> although I still think that having !d->arch.vtsc would be better, even 
> if not strictly required.

Since Haozhong agreed, and for symmetry with patch 1 I'm going to
commit with this added, plus ...

>> @@ -1780,9 +1783,9 @@ void tsc_get_info(struct domain *d, uint32_t *tsc_mode,
>>           else
>>           {
>>               tsc = rdtsc();
>> -            *elapsed_nsec = scale_delta(tsc, &d->arch.vtsc_to_ns) -
>> +            *elapsed_nsec = scale_delta(tsc, &this_cpu(cpu_time).tsc_scale) 
>> -
>>                               d->arch.vtsc_offset;
>> -            *gtsc_khz = 0; /* ignored by tsc_set_info */
>> +            *gtsc_khz = enable_tsc_scaling ? d->arch.tsc_khz : 0;

... with the comment here retained.

Jan


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