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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

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This number might be out of date as soon as it is read, won't it ?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?
> 
> 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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