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Subject: Re: [hwloc-devel] hwloc with Xen system support - v2
From: Andrew Cooper (andrew.cooper3_at_[hidden])
Date: 2014-01-07 09:19:26

On 07/01/14 11:54, Brice Goglin wrote:
> Hello,
> Can't check the code right now, but a couple questions below.
> One of the issues I had was that the Core IDs (as reported by Xen)
> are enumerated per socket rather than as an entire system. The
> purpose of "HACK - patch up cpu_to_core." in hwloc_get_xen_info()
> is to change the per-socket enumeration to being per system.
> Samuel believes that hwloc should be able to cope with duplicate
> core IDs with different cpusets, but if I attempt to do that, I
> get the following error:
> /****************************************************************************/
> /* hwloc has encountered what looks like an error from the operating system./
> /*/
> /* object (Core P#0 cpuset 0x30000003) intersection without inclusion!/
> /* Error occurred in topology.c line 853/
> /*/
> /* Please report this error message to the hwloc user's mailing list,/
> /* along with the output from the script./
> /****************************************************************************/
> I don't understand what's going on here. Can you post the list of
> PU/core/socket IDs that Xen reports so that I see what is unique and
> what is not?

I will get back to you with the exact ids from the problematic system.

> I currently have a crazy idea for getting at the cache
> information. topology-x86.c has a lot of cpuid knowledge, and I
> have a proposed new hypercall which executes cpuid on a specific
> PU. Would it be possible (or indeed sensible) to parametrise the
> code in topology-x86.c to take a few function pointers for get/set
> binding information, and for the cpuid call itself?
> I don't see why we couldn't do that. Can you post an example of what
> the Xen cpuid hypercall prototype would be, so that I see how I need
> to change the x86 backend?

I have already (pre-emptively) started hacking on the code. I shall see
if I can get an RFC patch ready, but I am intending to do it will very
few changes to the cpuid code.

> That way, the common x86 knowledge could be used correctly by the
> Xen component, while still keeping its current design.
> Is there anything that the current Xen backend supports and that
> wouldn't be feasible through x86 cpuid? The x86 component can already
> detect a lot of topology information, including
> cores/caches/sockets/NUMA. Maybe the NUMA node sizes?
> By the way, which architectures are supported by Xen aside of x86?
> Does Xen have topology information for them?
> Brice

Xen currently supports x86 and has experimental support for arm32 and
arm64. The information available from the current code is generic,
should should JustWork even on the arm systems.

It would certainly be interesting to see whether my current Xen code
agrees with x86 cpuid's idea of the topology.