linux-kernel - Re: [Xen-devel] [PATCH][v9 4/6] xen/hvm: Xen PV extension of HVM initialization

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Message-Id: <201003160951.48973.sheng@linux.intel.com>
Date:	Tue, 16 Mar 2010 09:51:48 +0800
From:	Sheng Yang <sheng@...ux.intel.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	"xen-devel" <xen-devel@...ts.xensource.com>,
	Ian Campbell <Ian.Campbell@...citrix.com>,
	"Yaozu (Eddie) Dong" <eddie.dong@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ian Pratt <Ian.Pratt@...citrix.com>,
	Keir Fraser <Keir.Fraser@...citrix.com>
Subject: Re: [Xen-devel] [PATCH][v9 4/6] xen/hvm: Xen PV extension of HVM initialization

On Tuesday 16 March 2010 06:59:58 Jeremy Fitzhardinge wrote:
> On 03/15/2010 05:04 AM, Stefano Stabellini wrote:
> >> But we should make sure Xen have ability to support such kind of
> >> operation. The CPUID would show if Xen have such ability, and if it
> >> does, the feature would be enabled unconditionally. Guest kernel always
> >> enable all features it can do unconditionally, but Xen should offer the
> >> support for them.
> >
> > In my opinion once the guest knows that is running on Xen HVM (that is
> > from xen_cpuid_base() or xen_para_available()) it should assume
> > that the pv clocksource is available, therefore XEN_HVM_PV_CLOCK_ENABLED
> > should not be needed.
> > In other words the mere presence of Xen should imply
> > XEN_HVM_PV_CLOCK_ENABLED.
> 
> The only reason why we wouldn't want to do this is if we want to
> withdraw this feature at some point in the future.  We're stuck with it
> indefinitely for PV, but I don't know if that's necessarily going to be
> the case for HVM.  On the other hand, if other - better - mechanisms
> become available, we can give them their own clocksource driver with a
> higher priority than the Xen pvclock one, and users can still select
> clocksources on the kernel command line.

So you think about adding a new XENFEAT?
> 
> > Do you mean write generic code now, then introduce the 64 bit
> > limitation later? Or the other way around?
> > I don't have a strong opinion here so I am OK with both approaches, but
> > I would prefer to add the limitation later (maybe we'll be able to make
> > it work on 32 bit too...).
> 
> Seems like making it work for both 32 and 64-bit is the easiest thing to
>  do.

If it is, it should be fine. But I had encountered some issues on 32 bits.

-- 
regards
Yang, Sheng
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