Re: [Xen-devel] [PATCH 01/13] x86/time.c: Use system time to calculate elapsed_nsec in tsc_get_info()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.10.15 at 18:09, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 10/09/2015 11:11 AM, Jan Beulich wrote:
>>>>> On 09.10.15 at 16:00, <haozhong.zhang@xxxxxxxxx> wrote:
>>> On Fri, Oct 09, 2015 at 09:41:36AM -0400, Boris Ostrovsky wrote:
>>>> On 10/09/2015 02:51 AM, Jan Beulich wrote:
>>>>>>>> On 28.09.15 at 09:13, <haozhong.zhang@xxxxxxxxx> wrote:
>>>>>> When the TSC mode of a domain is TSC_MODE_DEFAULT and no TSC emulation
>>>>>> is used, the existing tsc_get_info() calculates elapsed_nsec by scaling
>>>>>> the host TSC with a ratio between guest TSC rate and
>>>>>> nanoseconds. However, the result will be incorrect if the guest TSC rate
>>>>>> differs from the host TSC rate. This patch fixes this problem by using
>>>>>> the system time as elapsed_nsec.
>>>>> For both this and patch 2, while at a first glance (and taking into
>>>>> account just the visible patch context) what you say seems to
>>>>> make sense, the explanation is far from sufficient namely when
>>>>> looking at the function as a whole. For one, effects on existing
>>>>> cases need to be explicitly described, in particular why SVM's TSC
>>>>> ratio code works without that change (or whether it has been
>>>>> broken all along, in which case these would become backporting
>>>>> candidates; input from SVM maintainers would be appreciated
>>>>> too). That may in particular mean being more specific about
>>>>> what is actually wrong with scaling the host TSC here (i.e. in
>>>>> which way both results differ), when supposedly that matches
>>>>> what the hardware does when TSC ratio is supported.
>>>> If elapsed_nsec is the time that guest has been running then how can
>>>> get_s_time(), which is system time, be the right answer here? But what
>>>> confuses me even more is that existing code is not doing that neither.
>>>>
>>>> Shouldn't elapsed_nsec be offset by d->arch.vtsc_offset on the get side?
>>>> I.e.
>>>>
>>>> *elapsed_nsec = get_s_time() - d->arch.vtsc_offset?
>>>>
>>> Yes, I should minus d->arch.vtsc_offset here.
>> In which case - afaict - the code becomes identical to that of the
>> TSC_MODE_ALWAYS_EMULATE case as well as the
>> TSC_MODE_DEFAULT w/ d->arch.vtsc true. Which seems quite
>> unlikely to be correct.
> 
> *elapsed_nsec = *gtsc_khz = 0; ? Because we are effectively in 
> TSC_MODE_NEVER.

How that? Talk here has been about TSC_MODE_DEFAULT...

> That can't be right...

Why not? tsc_set_info() doesn't care about any of its other input
values when that mode is in effect.

Jan


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