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

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Message-ID: <f4315e27ffc14047b5e2ef61eeeba036@SN2PR03MB061.namprd03.prod.outlook.com>
Date:	Tue, 29 Jan 2013 15:58:08 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	Olaf Hering <olaf@...fle.de>, Jan Beulich <JBeulich@...e.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] x86: Hyper-V: register clocksource only if its
 advertised



> -----Original Message-----
> From: Stefano Stabellini [mailto:stefano.stabellini@...citrix.com]
> Sent: Tuesday, January 29, 2013 10:31 AM
> To: KY Srinivasan
> Cc: Olaf Hering; Jan Beulich; Stefano Stabellini; Greg KH; linux-
> kernel@...r.kernel.org
> Subject: RE: [PATCH] x86: Hyper-V: register clocksource only if its advertised
> 
> On Tue, 29 Jan 2013, KY Srinivasan wrote:
> > > The patch which started this thread is still valid because it enables
> > > feature B only if the featurebit for B is enabled.
> >
> > Why would we need this if we have some other way of detecting that Hyper-V
> is being
> > emulated.
> 
> I don't think that Hyper-V being emulated or not matters here.
> What matters is whether the feature is available: only if it is then it
> should be used.
> 
> 
> > Furthermore, I am not sure I understand the logic here. The fact that the
> hypervisor
> > supports PARTITION_REFERENCE_COUNTER does not necessarily mean that we
> should register
> > a clocksource based on that reference counter. It is true that that is the case on
> Hyper-V today.
> > However, on other hypervisors emulating Hyper-V, other standard
> clocksources maybe more
> > appropriate.
> 
> The point is that if a feature is NOT supported, then it should NOT be
> used. Only if it is supported, then Linux might consider using it. And
> of course can decide not to use it.
> 
> 
> > I agree with you that your patch is valid for making the Hyper-V code more
> robust but not for dealing
> > with general issue of Xen emulating Hyper-V.
> 
> What "general issue" would that be?

Not all Hyper-V specific features are advertised via a feature bit. For instance, we need to reserve
an IDT entry to have VMBUS interrupts delivered on a per-CPU basis and this reservation is done
after confirming that we are running on Hyper-V. With Xen emulating Hyper-V, we have an issue.
Since Hyper-V will never emulate Xen, I am going to add a check in the Hyper-V detection code to
deal with this issue.

Regards,

K. Y
 


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