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

To: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH][v9 4/6] xen/hvm: Xen PV extension of HVM initialization
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Mon, 15 Mar 2010 15:59:58 -0700
Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, "Yaozu \(Eddie\) Dong" <eddie.dong@xxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, Keir Fraser <Keir.Fraser@xxxxxxxxxxxxx>, Sheng Yang <sheng@xxxxxxxxxxxxxxx>
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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

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.

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.


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