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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 10/09/2015 12:51 PM, Haozhong Zhang wrote:
> 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 ?
Yes.

> 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.
I was trying to say that TSC behavior in current incarnation is 
equivalent to _NEVER if d->arch.vtsc is 0. But when we call 
tsc_get_info() we can not handle it out of _NEVER case (because, as you 
pointed out, d->arch.vtsc may change after migration). And we don't.
-boris


> - 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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