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

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Message-ID: <alpine.DEB.2.02.1301291522460.10432@kaball.uk.xensource.com>
Date:	Tue, 29 Jan 2013 15:30:51 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	KY Srinivasan <kys@...rosoft.com>
CC:	Olaf Hering <olaf@...fle.de>, Jan Beulich <JBeulich@...e.com>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.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

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?
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