Re: [Xen-devel] [PATCH v3 1/7] xen: vNUMA support for PV guests

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.11.13 at 17:36, Dario Faggioli <dario.faggioli@xxxxxxxxxx> wrote:
> On mar, 2013-11-19 at 15:54 +0000, Jan Beulich wrote:
>> >>> On 19.11.13 at 16:42, Dario Faggioli <dario.faggioli@xxxxxxxxxx> wrote:
>> > So, what would the best option be? Another hypercall (or a special way
>> > of calling this one) "just" to retrieve the number of vnodes?
>> 
>> Iirc there's a padding field in the interface structure, which could
>> be leveraged. But then again you need two counts, and hence it
>> might be better to simply add two respective fields. Then make
>> it/them IN/OUT, and rather than filling the arrays when they're
>> too small just send back the necessary values. (And of course
>> you'll want to also send back the actual values in case the passed
>> in ones turned out to large, so the guest would know how many
>> of the array elements actually have valid data).
>> 
>> But in the end the fundamental question stands - how was a PV
>> guest in your so far proposed model supposed to know its number
>> of vNodes? While for HVM guests you can make this available via
>> ACPI, that's not an option for PV.
>> 
> Wait... I'm no longer so sure I'm getting what you say. I'd be inclined
> to say "by the XENMEM_get_vnuma_info hcall implemented here", but then
> again, maybe I'm missing something.
> 
> The hypercall does provide a mean for the guest to retrieve _all_ the
> virtual topology information, such as:
>  - number of virtual nodes
>  - virtual node memory ranges
>  - virtual cpu to virtual node mapping
>  - virtual node to physical node mapping, for use in (future) in-guest 
>    vNUMA aware subsystems (e.g., ballooning)
> 
> So, if your point is (as I thought) that for properly allocating the
> buffers for this hypercall to work we need an information only provided
> by this hypercall itself, then I agree, and that's why I asked what
> alternative way would be best to retrieve that bit of information.
> 
> If it's something else, then I don't know. :-)

No, it is what you're naming above. I was merely curious how you
had supposed the guest would know the vNode count prior to this
change request of mine. I didn't look at the Linux patches yet (due
to lack of time), hence I don't know how you derived(?) the node
count without it coming back from the hypercall here.

Jan


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