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Re: [Xen-devel] Fam10+ extended config space enabling

To: "Keir Fraser" <keir@xxxxxxx>
Subject: Re: [Xen-devel] Fam10+ extended config space enabling
From: "Jan Beulich" <JBeulich@xxxxxxxxxx>
Date: Thu, 06 Jan 2011 11:17:11 +0000
Cc: robert.richter@xxxxxxx, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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>>> On 06.01.11 at 11:38, Keir Fraser <keir@xxxxxxx> wrote:
> On 06/01/2011 10:15, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:
>> Robert,
>> starting with 2.6.27 Linux is doing this, using MSR accesses (these
>> days in CPU hotplug notification callbacks). Is there a reason this
>> cannot be done through PCI config space writes (as the register is
>> aliased), and then only once at boot time (perhaps utilizing some
>> of the code in arch/x86/kernel/k8.c)?
>> I'm particularly asking from the Xen perspective, where the MSR
>> based mechanism won't reliably work (since there's no guarantee
>> all of Dom0's virtual CPUs would, at initialization time, actually
>> cover all physical CPUs that Dom0 may ever run on).
> Just thinking, could we simply do this in Xen, either unconditionally, or as
> soon as we detect dom0 doing it for at least one CPU?

We certainly could (albeit not unconditional I'd say; this was also
my first thought, with the alternative of adding a specific
hypercall), but it would mean adding special code for something
that can be done in Dom0 (and there even in a way that's suitable
for both native and Xen) if using the PCI config space method for
this is possible.


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