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

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Message-Id: <510797FA02000078000BA4B8@nat28.tlf.novell.com>
Date:	Tue, 29 Jan 2013 08:35:54 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Stefano Stabellini" <stefano.stabellini@...citrix.com>,
	"KY Srinivasan" <kys@...rosoft.com>
Cc:	"Olaf Hering" <olaf@...fle.de>,
	"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 28.01.13 at 18:44, Stefano Stabellini <stefano.stabellini@...citrix.com> wrote:
> I think that Olaf made his point very clear: a feature A should only be
> enabled if the corresponding flag A is set.
> In fact it seems to me that this patch is correct on its own merits,
> regardless of Xen does or does not.
> 
> The Xen tools might or might not know whether a guest is going to be
> Linux, Windows, FreeBSD or whatever else people use nowadays.  Setting
> viridian=1 is the safe choice, given that it shouldn't create any
> issues: after all guests are supposed to check for feature flags before
> using them.
> 
> If Xen is going to implement "Partition Reference Counter", it is also
> going to set the corresponding flag, so the guest OS (Windows, Linux,
> my pet OS) can check whether the feature is available and decide whether
> it wants to use it.

While I agree in general, the specific case of the callback vector
seems a little more difficult: As KY says, there's no feature flag
for this (or perhaps more precisely for it being deliverable across
all CPUs), and hence there's both the problem of detection and
the problem of disambiguation (as otherwise both the Hyper-V
code and the Xen code in Linux could be trying to use the same
vector).

Jan

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