Re: [PATCH 09/13] x86/time.c: Scale host TSC in pvclock properly

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 10/22/2015 12:44 PM, Haozhong Zhang wrote:
> On Thu, Oct 22, 2015 at 11:50:18AM -0400, Boris Ostrovsky wrote:
>> On 09/28/2015 03:13 AM, 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@intel.com>
>>> ---
>>>   xen/arch/x86/time.c | 15 ++++++++++++---
>>>   1 file changed, 12 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
>>> index 4b5402c..54eab6e 100644
>>> --- a/xen/arch/x86/time.c
>>> +++ b/xen/arch/x86/time.c
>>> @@ -832,10 +832,19 @@ static void __update_vcpu_system_time(struct vcpu *v, int force)
>>>       }
>>>       else
>>>       {
>>> -        _u.tsc_timestamp     = t->local_tsc_stamp;
>>> +        if ( is_hvm_domain(d) && hvm_funcs.tsc_scaling_supported )
>>> +        {
>>> +            _u.tsc_timestamp     = hvm_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
>>> +        {
>>> +            _u.tsc_timestamp     = t->local_tsc_stamp;
>>> +            _u.tsc_to_system_mul = t->tsc_scale.mul_frac;
>>> +            _u.tsc_shift         = (s8)t->tsc_scale.shift;
>>> +        }
>>>           _u.system_time       = t->stime_local_stamp;
>>> -        _u.tsc_to_system_mul = t->tsc_scale.mul_frac;
>>> -        _u.tsc_shift         = (s8)t->tsc_scale.shift;
>>>       }
>>>       if ( is_hvm_domain(d) )
>>>           _u.tsc_timestamp += v->arch.hvm_vcpu.cache_tsc_offset;
>               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 	     the offset is subtract here

Ah, I missed this. Thanks.

-boris


>> So this is not directly related to this series but when we calculate
>> tsc_timestamp --- shouldn't we subtract TSC offset? Otherwise we are
>> reporting (possibly scaled) host's TSC and this is supposed to be guest's
>> counter.
>>
>>
>> -boris