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Re: [Xen-devel] [Patch] use full-size cpumask for vcpu-pin

To: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [Patch] use full-size cpumask for vcpu-pin
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Fri, 13 Aug 2010 14:15:45 +0100
Cc: Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On 13/08/2010 14:11, "Ian Jackson" <Ian.Jackson@xxxxxxxxxxxxx> wrote:

> Keir Fraser writes ("Re: [Xen-devel] [Patch] use full-size cpumask for
> vcpu-pin"):
>> Actually, now I look at it, I have to NACK the patch. Sorry I didn't look
>> closely enough earlier. I think the bug can be addressed without any
>> hypervisor changes: when vcpu-pinning, the tools can quite correctly pass a
>> cpumask to Xen just big enough to express just the CPUs in the new affinity
>> set. If the resulting mask is too narrow to address all CPUs in the system,
>> then Xen will pad it out with zeroes. If the resulting mask is too wide, Xen
>> will simply truncate it. All this is done silently at the time of the
>> setvcpuaffinity hypercall.
> The difficulty is, as I understand it, how to express the mask "all
> CPUs present and future".  Since the number of pcpus may be very
> large.  The tools presumably shouldn't pass a kilobyte of 0xff :-).

Since Xen discards any CPUs which are not online at the time of the
setvcpuaffinity hypercall, expressing "all CPUs present and future" is a bit

 -- Keir

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