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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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? I.e. why would you want to
tell the guest it's NUMA when it really isn't? The only case I could
see is testing vNUMA code changes without having a NUMA box
around, but that hardly counts as a real use.

Furthermore iirc that array is an array of uint32_t, and the
sentinel (if any) there ought to be 0xffffffff irrespective of what
we use internally in the hypervisor.

Jan


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