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

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

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@intel.com>
>>> ---
>>> 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.
>>
>> -boris
>>
> 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@oracle.com>


>
> Haozhong
>
>>>   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 force)
>>>       }
>>>       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;
>>> +        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;