Re: [Xen-devel] Crash in set_cpu_sibling_map() booting Xen 4.6.0 on Fusion

[Ed Swierk <eswierk@xxxxxxxxxxxxxxxxxx>]

On Tue, Nov 24, 2015 at 2:34 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> Indeed, and I think I had said so. The algorithm does, however, tell
> us that with the above output CPU 3 (APIC ID 6) is on socket 6 (both
> shifts being zero), which for the whole system results in sockets 1,
> 3, and 5 unused. While not explicitly excluded, I'm not sure how far
> we should go in expecting all kinds of odd configurations (along those
> lines we e.g. have a limit on the largest APIC ID we allow: MAX_APICS /
> MAX_LOCAL_APIC, which for big systems is 4 times the number of
> CPUs we support).

That's why I thought it reasonable to substitute MAX_APICS for
nr_sockets in sizing the socket_cpumask array.

> Taking it to set_nr_sockets(), a pretty basic assumption is broken by
> the above way of presenting topology: We would have to have more
> sockets than there are CPUs. I would have wanted to check what
> e.g. Linux does here, but there doesn't seem to be any support of
> CAT (and hence any need for per-socket data) there.

I looked at Linux, and there is no per-socket bookkeeping, AFAICT.

> (I am, btw, now also confused by you saying that e.g. for a 3-CPU
> config things work. If the topology data gets presented in similar
> ways in that case, I can't see why you wouldn't run into the same
> problem. Unless memory corruption occurs silently in one case, but
> "loudly" in the other.)

For 3, 6 and 12 CPUs, Fusion presents a completely different topology,
with 3-core sockets numbered consecutively starting with 0.

> Bottom line - for the moment I do not see a reasonable way of
> dealing with that situation. The closest I could see would be what
> we iirc had temporarily during the review cycles of the initial CAT
> series: A command line option to specify the number of sockets. Or
> make all accesses to socket_cpumask[] conditional upon PSR being
> enabled (which would have the bad side effect of making future
> uses for other purposes more cumbersome), or go through and
> range check the socket number on all of those accesses.

Could we avoid the issue by replacing socket_cpumask array with a list
or hashtable, indexed by socket ID?

> Chao, could you - inside Intel - please check whether there are
> any assumptions on the respective CPUID leaf output that aren't
> explicitly stated in the SDM right now (like resulting in contiguous
> socket numbers), and ask for them getting made explicit (if there
> are any), or it being made explicit that no assumptions at all are
> to be made at all on the presented values (in which case we'd
> have to consume MADT parsing data in set_nr_sockets(), e.g.
> by replacing num_processors there with one more than the
> maximum APIC ID of any non-disabled CPU)?

I suppose the key is whether Intel has encoded such assumptions in the
BIOS reference code, or has otherwise communicated them to AMI et al.

--Ed

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