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 12:31:53PM -0400, Boris Ostrovsky wrote:
> On 10/09/2015 12:19 PM, Jan Beulich wrote:
> >>>>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...
> 
> AFAIUI, TSC_MODE_DEFAULT is a shorthand for saying "I will let the
> hypervisor pick whether the guest will be in TSC_MODE_ALWAYS_EMULATE or
> TSC_MODE_NEVER". d->arch.vtsc is what ends up being internal implementation
> of user-provided mode (for the most parts; I think hvm_cpuid() being the
> only true exception --- and perhaps it needs to be looked at).
> 
> So if we have d->arch.vtsc=0 (which is the case we are talking about here)
> then we are really in NEVER mode
>

Not quite understand this. Is tsc_set_info() the only place to set
d->arch.tsc_mode ? Though it may decide d->arch.vtsc should be 1, it
still sets d->arch.tsc_mode to the user provided TSC mode for a
non-pvh domain. And then in tsc_get_info(), it should never fall into
TSC_MODE_NEVER_EMULATE branch if d->arch.tsc_mode is not.

- Haozhong

> 
> -boris
> 
> >
> >>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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