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

[Haozhong Zhang <haozhong.zhang@xxxxxxxxx>]

On Fri, Oct 09, 2015 at 11:37:06AM -0400, Boris Ostrovsky wrote:
> On 10/09/2015 10:39 AM, Jan Beulich wrote:
> >>>>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?
> 
> We only use elapsed_nsec when vtsc is set, I think. In native case (vtsc=0)
> elapsed_nsec and d->arch.vtsc_offset are ignored.
>

But it is used in tsc_set_info() if a HVM domain in TSC_MODE_DEFAULT
is migrated to a machine and the following if condition in
tsc_set_info() is false.

if ( tsc_mode == TSC_MODE_DEFAULT && host_tsc_is_safe() &&
     (has_hvm_container_domain(d) ?
      d->arch.tsc_khz == cpu_khz || cpu_has_tsc_ratio :
      incarnation == 0) )

- Haozhong

> -boris
> 
> 

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