linux-kernel - RE: [PATCH] x86: Hyper-V: register clocksource only if its advertised

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Message-ID: <500ca90f5d3e4d4f80c7fb62c34d36f0@BLUPR03MB050.namprd03.prod.outlook.com>
Date:	Fri, 25 Jan 2013 17:04:09 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	Olaf Hering <olaf@...fle.de>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	"Jan Beulich (JBeulich@...e.com)" <JBeulich@...e.com>
Subject: RE: [PATCH] x86: Hyper-V: register clocksource only if its
 advertised



> -----Original Message-----
> From: Olaf Hering [mailto:olaf@...fle.de]
> Sent: Friday, January 25, 2013 11:55 AM
> To: KY Srinivasan
> Cc: linux-kernel@...r.kernel.org; Greg KH
> Subject: Re: [PATCH] x86: Hyper-V: register clocksource only if its advertised
> 
> On Fri, Jan 25, KY Srinivasan wrote:
> 
> >
> >
> > > -----Original Message-----
> > > From: Olaf Hering [mailto:olaf@...fle.de]
> > > Sent: Friday, January 25, 2013 8:37 AM
> > > To: linux-kernel@...r.kernel.org
> > > Cc: Olaf Hering; stable@...nel.org; KY Srinivasan; Greg KH
> > > Subject: [PATCH] x86: Hyper-V: register clocksource only if its advertised
> > >
> > > Enable hyperv_clocksource only if its advertised as a feature.
> > > XenServer 6 returns the signature which is checked in
> > > ms_hyperv_platform(), but it does not offer all features. Currently the
> > > clocksource is enabled unconditionally in ms_hyperv_init_platform(), and
> > > the result is a hanging guest.
> > >
> > > Hyper-V spec Bit 1 indicates the availability of Partition Reference
> > > Counter.  Register the clocksource only if this bit is set.
> > >
> > > The guest in question prints this in dmesg:
> > >  [    0.000000] Hypervisor detected: Microsoft HyperV
> > >  [    0.000000] HyperV: features 0x70, hints 0x0
> > >
> > > This bug can be reproduced easily be setting 'viridian=1' in a HVM domU
> > > .cfg file. A workaround without this patch is to boot the HVM guest with
> > > 'clocksource=jiffies'.
> >
> > I am curious why you are setting viridian=1 for a Linux HVM guest. Initially when
> > I did the Hyper-V emulation work on Xen, this configuration flag would be set
> only
> > for Windows guests since the idea was to run the "enlightened" Windows on
> Xen.
> > While the current Xen emulation code does not emulate "Partition Reference
> Counter"
> > there is no guarantee that Xen would not do this in the future. Perhaps dealing
> with this issue
> > by identifying the guest (via the viridian domain or not) may be a safer
> approach?
> 
> I'm not sure if XenServer does that per default, but I used the
> viridian= flag to reproduce the report.
> If Xen starts to emulate the "Partition Reference Counter" it should
> also advertise this emulation with the feature flag. Are you saying that
> checking for features is not the right approach?

My fear is that there is no guarantee that Xen would not emulate this feature
in the spirit of making Hyper-V emulation "more" complete. Since all this problem is
because Xen thinks it is running a viridian domain (when in fact it is running Linux), I felt we
should explore why the viridian tag got set for a Linux VM. If we can fix that we would have
a solution that does not depend upon assuming that Xen would not emulate a particular Hyper-V
feature.

K. Y