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Re: [Xen-devel] [PATCH v4 03/15] libxl: introduce libxl_get_nr_cpus()
- To: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>, Dario Faggioli <dario.faggioli@xxxxxxxxxx>
- From: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
- Date: Tue, 3 Dec 2013 18:09:10 +0000
- Cc: Marcus Granado <Marcus.Granado@xxxxxxxxxxxxx>, Keir Fraser <keir@xxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxx>, Li Yechen <lccycc123@xxxxxxxxx>, Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>, Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxx, Jan Beulich <JBeulich@xxxxxxxx>, Justin Weaver <jtweaver@xxxxxxxxxx>, Matt Wilson <msw@xxxxxxxxxx>, Elena Ufimtseva <ufimtseva@xxxxxxxxx>
- Delivery-date: Tue, 03 Dec 2013 18:10:32 +0000
- List-id: Xen developer discussion <xen-devel.lists.xen.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This number might be out of date as soon as it is read, won't it ?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
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