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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
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