Re: [Xen-devel] [PATCH v2 14/14] docs: Add descriptions of TSC scaling in xl.cfg and tscmode.txt

["Zhang, Haozhong" <haozhong.zhang@xxxxxxxxx>]

On 12/10/15 18:40, Tian, Kevin wrote:
> > From: Zhang, Haozhong
> > Sent: Monday, December 07, 2015 4:59 AM
> g and tscmode.txt
> > 
> > Signed-off-by: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>
> > ---
> >  docs/man/xl.cfg.pod.5 | 15 ++++++++++++++-
> >  docs/misc/tscmode.txt | 14 ++++++++++++++
> >  2 files changed, 28 insertions(+), 1 deletion(-)
> > 
> > diff --git a/docs/man/xl.cfg.pod.5 b/docs/man/xl.cfg.pod.5
> > index 2aca8dd..7e19a9b 100644
> > --- a/docs/man/xl.cfg.pod.5
> > +++ b/docs/man/xl.cfg.pod.5
> > @@ -1313,9 +1313,18 @@ deprecated. Options are:
> > 
> >  =item B<"default">
> > 
> > -Guest rdtsc/p executed natively when monotonicity can be guaranteed
> > +Guest rdtsc/p is executed natively when monotonicity can be guaranteed
> >  and emulated otherwise (with frequency scaled if necessary).
> > 
> > +If a HVM container in B<default> TSC mode is not migrated from other hosts
> 
> "migrated from" -> "migrated to"?
>

I mean "migrated from" here. If the current host supports TSC scaling
and a domain is migrated from another host w/ different host TSC
frequency, then domain may have a different guest TSC frequency than
the current host. Thus, "not migrated from other hosts" is used here
to eliminate such case.

> > +and the host TSC monotonicity can be guaranteed, the guest and host TSC
> > +frequencies will be the same.
> > +
> > +If a HVM container in B<default> TSC mode is migrated to a host that can
> > +guarantee the TSC monotonicity and supports Intel VMX TSC scaling/AMD SVM
> 
> and -> or? Do we think TSC scaling a must to ensure TSC monotonicity? It comes
> to the rescue only when host can't ensure monotonicity...
>

No, I intend to describe the guest behavior when hardware TSC scaling is used.

Really I should say "_host_ TSC monotonicity" here.

> > +TSC ratio, guest rdtsc/p will still execute natively after migration and 
> > the
> > +guest TSC frequencies before and after migration will be the same.
> 
> will be the same before and after migration.
>

will modify in the next version.

> > +
> >  =item B<"always_emulate">
> > 
> >  Guest rdtsc/p always emulated at 1GHz (kernel and user). Guest rdtsc/p
> > @@ -1337,6 +1346,10 @@ determine when a restore/migration has occurred and
> > assumes guest
> >  obtains/uses pvclock-like mechanism to adjust for monotonicity and
> >  frequency changes.
> > 
> > +If a HVM container in B<native_paravirt> TSC mode can execute both guest
> > +rdtsc and guest rdtscp natively, then the guest TSC frequency will be
> > +determined in the similar way to that of B<default> TSC mode.
> > +
> >  =back
> > 
> >  Please see F<docs/misc/tscmode.txt> for more information on this option.
> > diff --git a/docs/misc/tscmode.txt b/docs/misc/tscmode.txt
> > index e8c84e8..f3b70be 100644
> > --- a/docs/misc/tscmode.txt
> > +++ b/docs/misc/tscmode.txt
> > @@ -297,3 +297,17 @@ and also much faster than nearly all OS-provided time
> > mechanisms.
> >  While pvrtscp is too complex for most apps, certain enterprise
> >  TSC-sensitive high-TSC-frequency apps may find it useful to
> >  obtain a significant performance gain.
> > +
> > +Hardware TSC Scaling
> > +
> > +Intel VMX TSC scaling and AMD SVM TSC ratio allow the guest TSC read
> > +by guest rdtsc/p increasing in the different frequency than the host
> 
> "in the different" -> "in a different"
>

will modify

> > +TSC frequency.
> > +
> > +For a HVM container is in default TSC mode (tsc_mode=0) or PVRDTSCP
> 
> For a HVM container *which* is
>

stupid error... will modify

> > +mode (tsc_mode=3) and can execute both guest rdtsc and rdtscp
> > +natively, if it is not migrated from other hosts, the guest and host
> > +TSC frequencies will be the same. 
> 
> "the guest and host TSC frequencies remain the same if the guest is
> not migrated to other host."
> 
> and the condition is that the host supports constant TSC feature.
>

Yes, I'll modify in the next version.

Thanks,
Haozhong

> > If it is migrated to a host
> > +supporting Intel VMX TSC scaling/AMD SVM TSC ratio and can still
> > +execute guest rdtsc and rdtscp natively, the guest TSC frequencies
> > +before and after migration will be the same.
> > --
> > 2.6.3
> 

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel