Re: [Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 07/16/2015 11:45 AM, Andrew Cooper wrote:
> On 16/07/15 16:25, Wei Liu wrote:
> > On Thu, Jul 16, 2015 at 11:56:50AM +0100, Andrew Cooper wrote:
> > > On 16/07/15 11:47, Jan Beulich wrote:
> > > > > > > On 16.07.15 at 12:32, <dario.faggioli@xxxxxxxxxx> wrote:
> > > > > root@test:~# numactl --hardware
> > > > > available: 2 nodes (0-1)
> > > > > node 0 cpus: 0 1
> > > > > node 0 size: 475 MB
> > > > > node 0 free: 382 MB
> > > > > node 1 cpus: 2 3
> > > > > node 1 size: 495 MB
> > > > > node 1 free: 475 MB
> > > > > node distances:
> > > > > node   0   1
> > > > >    0:  10  10
> > > > >    1:  20  10
> > > > >
> > > > > root@test:~# cat /sys/devices/system/cpu/cpu0/topology/thread_siblings_list
> > > > > 0-1
> > > > > root@test:~# cat /sys/devices/system/cpu/cpu0/topology/core_siblings_list
> > > > > 0-3
> > > > > root@test:~# cat /sys/devices/system/cpu/cpu2/topology/thread_siblings_list
> > > > > 2-3
> > > > > root@test:~# cat /sys/devices/system/cpu/cpu2/topology/core_siblings_list
> > > > > 0-3
> > > > >
> > > > > So the complain during boot seems to be against 'core_siblings' (which
> > > > > was not what I expected, but perhaps I misremember the meaning of
> > > > > "core_siblings" VS. "thread_siblings" VS. smt-siblings in Linux; I'll
> > > > > double check).
> > > > >
> > > > > Anyway, is there anything we can do to fix or workaround things?
> > > > Make the guest honor topology also at the CPUID layer. Whether
> > > > that's by not wrongly consuming the respective CPUID bits (i.e. a
> > > > guest side change) or reflecting PV state in what the hypervisor
> > > > returns I'm not sure about. While the latter might be more clean,
> > > > I'd be afraid this might get in the way of what the tool stack wants
> > > > to see.
> > > Xen's CPUID handling currently has no concept of per-core and
> > > per-package data in the cpuid policy.  The guest sees the information
> > Can / Will Xen have that concept in the future?
> It is certainly possible.
>
> I plan to try and lay some ground work as part of the feature levelling
> fixes, but fixing the hypervisor representation of cpuid is specifically
> out of scope for the feature levelling fixes (in a deliberate attempt to
> prevent the project expanding to fill more time than I have).
>
> It is on my list of areas to tackle, but it is several nested cans of worms.
Can't we set leaf 1's EBX[32:16] to 1?

-boris


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