Re: [Xen-devel] [PATCH v6 02/23] xen: move NUMA_NO_NODE to public memory.h as XEN_NUMA_NO_NODE

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 02/03/15 17:26, Jan Beulich wrote:
>>>> On 02.03.15 at 18:01, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 02/03/15 16:50, Jan Beulich wrote:
>>>>>> On 02.03.15 at 17:39, <wei.liu2@xxxxxxxxxx> wrote:
>>>> On Mon, Mar 02, 2015 at 04:27:25PM +0000, Jan Beulich wrote:
>>>>>>>> On 02.03.15 at 17:08, <wei.liu2@xxxxxxxxxx> wrote:
>>>>>> On Mon, Mar 02, 2015 at 03:51:37PM +0000, Jan Beulich wrote:
>>>>>>>>>> On 02.03.15 at 16:38, <wei.liu2@xxxxxxxxxx> wrote:
>>>>>>>> On Mon, Mar 02, 2015 at 03:30:21PM +0000, Ian Campbell wrote:
>>>>>>>>> On Mon, 2015-03-02 at 07:04 +0000, Jan Beulich wrote:
>>>>>>>>>>>>> Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 02/27/15 5:58 PM >>>
>>>>>>>>>>> On 27/02/15 16:51, Wei Liu wrote:
>>>>>>>>>>>> During last round review, Andrew wanted me to move this to Xen 
>>>>>>>>>>>> public
>>>>>>>>>>>> header to avoid reinventing it in libxc. Now this value is used in 
>>>>>>>>>>>> libxc
>>>>>>>>>>>> patch.
>>>>>>>>>>>>
>>>>>>>>>>>> But I don't particularly mind whether we move it or not, it's up 
>>>>>>>>>>>> to you
>>>>>>>>>>>> maintainers to decide.
>>>>>>>>>>> It is a sentinel value used in the public ABI.  It should therefore
>>>>>>>>>>> appear in the public API.
>>>>>>>>>> Which it already does, as XENMEMF_get_node(0). I don't think it needs
>>>>>>>>>> particular naming as a new constant, even more that it isn't 
>>>>>>>>>> intended to
>>>>>>>>>> be used explicitly in any of the memops.
>>>>>>>>> IMHO the named constant does seem to make the tools code at least more
>>>>>>>>> readable, but without Wei having said where this is to be used I'm not
>>>>>>>>> sure where it should live. In particular I'm unsure if/how/where this
>>>>>>>>> value gets passed to a hypercall, as opposed to perhaps being used as 
>>>>>>>>> a
>>>>>>>> This is used to fill in vnode_to_pnode array. That array get
>>>>>>>> subsequently passed down to hypervisor.
>>>>>>> Do we really accept NUMA_NO_NODE to be passed that way?
>>>>>>>
>>>>>> public/domctl.h:struct xen_domctl_vnuma has vnode_to_pnode array.
>>>>> That wasn't my concern - I was rather wondering why we would
>>>>> accept any of this array's fields to be set to "no node".
>>>>>
>>>> If you want to have numa topology exposed to guest but doesn't care
>>>> about underly memory affinity?
>>> Is this useful for anything in reality?
>>
>> Yes.  If there is insufficient memory on real numa nodes, the memory
>> could be striped and the guest told that its memory really is scattered
>> all over.
> 
> I which case it could as well not be handed any vNUMA info at all.

A guest that previously had 2 vNUMA nodes is migrated to a host with
only 1 pNUMA node.  It should still have 2 vNUMA nodes.

David

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