Re: [Xen-devel] [PATCH v4 03/15] libxl: introduce libxl_get_nr_cpus()

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Tue, Dec 3, 2013 at 6:17 PM, Konrad Rzeszutek Wilk
<konrad.wilk@xxxxxxxxxx> wrote:
> On Tue, Dec 03, 2013 at 06:09:10PM +0000, George Dunlap wrote:
>> 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This number might be out of date as soon as it is read, won't it ?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].
>
> What about the inverse? 2 PCPUs and 16 VCPUS? Won't that still throw
> this calculation off?

Both things being compared are pcpus.

 -George

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