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 15:41, <boris.ostrovsky@xxxxxxxxxx> 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?

Doesn't whether or not to adjust be the offset depend on d-arch.vtsc?

Jan


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