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[Xen-devel] AMD EPYC Topology problems
- To: Xen-devel List <xen-devel@xxxxxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Sun, 2 Dec 2018 20:23:05 +0000
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- Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, SuraveeSuthikulpanit <suravee.suthikulpanit@xxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Brian Woods <brian.woods@xxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Brijesh Singh <brijeshkumar.singh@xxxxxxx>
- Delivery-date: Sun, 02 Dec 2018 20:23:13 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
Hello,
I have dual socket server with the following processor:
[root@xrtmia-09-01 ~]# head /proc/cpuinfo
processor : 0
vendor_id : AuthenticAMD
cpu family : 23
model : 1
model name : AMD EPYC 7281 16-Core Processor
stepping : 2
Which has highlighted a issue in the topology derivation logic.
(Actually, it was discovered with Xen, but we share the same
topology infrastructure and the issue is also present with Linux).
There are a total of 64 threads in the system, made of two 32-thread
sockets. The APIC IDs for this system are sparse - they are
0x0-0x3, 0x8-0xb, 0x10-0x13 etc, all the way up to 0x7b.
This is because the socket is made of 4 nodes with 4 cores each, but
space has been left in the layout for the maximum possible number of
APIC IDs.
In particular, CPUID 0x80000008:ecx reports 0x0000601f. That is, an
APIC ID shift of 6 (reporting a maximum of 64 threads per socket),
and NC as 31 (reporting 32 threads per socket in the current
configuration).
c->x86_max_cores is derived from NC and shifted once to exclude
threads, giving it a final value of 16 cores per socket.
Given the sparseness of the APIC IDs, it is unsafe to allocate an
array of c->x86_max_cores entries, then index it with
c->cpu_core_id, as half the cores in the system have a
cpu_core_id greater than x86_max_cores. There is no logical core ID
derived during boot which might be a safe to use as an index.
Furthermore, the documentation indicates that these values are
expected to be per-package, while they are all actually per-socket
(with up to 4 nodes per socket) in the EPYC case.
In the short term, my fix will be to not use c->x86_max_cores for
sizing the array, but I think this discovery warrants a discussion
as to whether the current topology infrastructure/expectations are
suitable.
Thanks,
~Andrew
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