Re: [Xen-devel] [PATCH v2 2/4] sysctl: Make XEN_SYSCTL_topologyinfo sysctl a little more efficient

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 06.01.15 at 14:41, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 06/01/15 02:18, Boris Ostrovsky wrote:
>> Instead of copying data for each field in xen_sysctl_topologyinfo separately
>> put cpu/socket/node into a single structure and do a single copy for each
>> processor.
>>
>> There is also no need to copy whole op to user at the end, max_cpu_index is
>> sufficient
>>
>> Rename xen_sysctl_topologyinfo and XEN_SYSCTL_topologyinfo to reflect the 
> fact
>> that these are used for CPU topology. Subsequent patch will add support for
>> PCI topology sysctl.
>>
>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> 
> If we are going to change the hypercall, then can we see about making it
> a stable interface (i.e. not a sysctl/domctl)?  There are non-toolstack
> components which might want/need access to this information.  (i.e. I am
> still looking for a reasonable way to get this information from Xen in
> hwloc)

Now having mentioned the alternative of doing this via other than
sysctl several times, I started wondering why exactly we'd want
this: hwloc is still user mode code, i.e. whether you call this part of
"the toolstack" is ambiguous. The only real reason to make an
interface like this other than a sysctl would be if the kernel had a
need to access it. The stability of the interface otoh has no meaning
here at all imo. We could declare a particular sysctl as "will never
change", but I understand that the need to pass the correct
XEN_SYSCTL_INTERFACE_VERSION into the hypercall would still
make it a little cumbersome to use.

Jan


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