From mboxrd@z Thu Jan  1 00:00:00 1970
From: George Dunlap <george.dunlap@eu.citrix.com>
Subject: Re: [PATCH v4 03/15] libxl: introduce
	libxl_get_nr_cpus()
Date: Tue, 3 Dec 2013 18:09:10 +0000
Message-ID: <529E1E46.2060106@eu.citrix.com>
References: <20131122183332.11200.20231.stgit@Solace>	<20131122185658.11200.13074.stgit@Solace>	<21150.6530.28766.823281@mariner.uk.xensource.com>	<1386093157.5338.337.camel@Solace>
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To: Ian Jackson <Ian.Jackson@eu.citrix.com>, Dario Faggioli <dario.faggioli@citrix.com>
Cc: Marcus Granado <Marcus.Granado@eu.citrix.com>, Keir Fraser <keir@xen.org>, Ian Campbell <Ian.Campbell@citrix.com>, Li Yechen <lccycc123@gmail.com>, Andrew Cooper <Andrew.Cooper3@citrix.com>, Juergen Gross <juergen.gross@ts.fujitsu.com>, xen-devel@lists.xen.org, Jan Beulich <JBeulich@suse.com>, Justin Weaver <jtweaver@hawaii.edu>, Matt Wilson <msw@amazon.com>, Elena Ufimtseva <ufimtseva@gmail.com>
List-Id: xen-devel@lists.xenproject.org

On 12/03/2013 05:54 PM, Ian Jackson wrote:
> Dario Faggioli writes ("Re: [PATCH v4 03/15] libxl: introduce libxl_get_nr_cpus()"):
>> On mar, 2013-12-03 at 17:48 +0000, Ian Jackson wrote:
>>> This number might be out of date as soon as it is read, won't it ?
>>
>> Quite possible, yes.
>>
>> So, are you suggesting that we shouldn't even allow the user to read it?
>> Or that I should mention that in the comment? (Or something else?)
>
> Perhaps I didn't explain my concerns clearly enough.
>
> I wonder what is it for ?  Isn't it difficult to use correctly ?

Dario uses it in the new version of libxl_vcpu_set_affinity() to limit 
what was considered an "unreachable cpu" in its warning.  (v5 14/17) 
That is, if you set the affinity to 1111111111111111111111..., and there 
are only 4 pcpus, it will return 111100000.....  These don't match, and 
yet there are no unreachable cpus.  So it asks nr_cpus first, then only 
compares bits 1..[nr_cpus-1].

I'm not sure how often that would change, but if there was a race, I 
think it would just be a spurious error message (or lack thereof).

  -George