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[Bug 1856335] Re: Cache Layout wrong on many Zen Arch CPUs
From: |
Babu Moger |
Subject: |
[Bug 1856335] Re: Cache Layout wrong on many Zen Arch CPUs |
Date: |
Mon, 23 Dec 2019 15:41:38 -0000 |
Damir,
We normally test Linux guests here. Can you please give me exact qemu command
line. Even the SMP parameters(sockets,cores,threads,dies) will also work. I
will try to recreate it locally first.
Give me example what works and what does not work.
I have recently sent few more patches to fix another bug. Please check if this
makes any difference.
https://patchwork.kernel.org/cover/11272063/
https://lore.kernel.org/qemu-devel/address@hidden/
This should apply cleanly on git://github.com/ehabkost/qemu.git (branch
x86-next)
Note: I will be on vacation until first week of Jan. Responses will be
delayed.
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https://bugs.launchpad.net/bugs/1856335
Title:
Cache Layout wrong on many Zen Arch CPUs
Status in QEMU:
New
Bug description:
AMD CPUs have L3 cache per 2, 3 or 4 cores. Currently, TOPOEXT seems
to always map Cache ass if it was an 4-Core per CCX CPU, which is
incorrect, and costs upwards 30% performance (more realistically 10%)
in L3 Cache Layout aware applications.
Example on a 4-CCX CPU (1950X /w 8 Cores and no SMT):
<cpu mode='custom' match='exact' check='full'>
<model fallback='forbid'>EPYC-IBPB</model>
<vendor>AMD</vendor>
<topology sockets='1' cores='8' threads='1'/>
In windows, coreinfo reports correctly:
****---- Unified Cache 1, Level 3, 8 MB, Assoc 16, LineSize 64
----**** Unified Cache 6, Level 3, 8 MB, Assoc 16, LineSize 64
On a 3-CCX CPU (3960X /w 6 cores and no SMT):
<cpu mode='custom' match='exact' check='full'>
<model fallback='forbid'>EPYC-IBPB</model>
<vendor>AMD</vendor>
<topology sockets='1' cores='6' threads='1'/>
in windows, coreinfo reports incorrectly:
****-- Unified Cache 1, Level 3, 8 MB, Assoc 16, LineSize 64
----** Unified Cache 6, Level 3, 8 MB, Assoc 16, LineSize 64
Validated against 3.0, 3.1, 4.1 and 4.2 versions of qemu-kvm.
With newer Qemu there is a fix (that does behave correctly) in using the dies
parameter:
<qemu:arg value='cores=3,threads=1,dies=2,sockets=1'/>
The problem is that the dies are exposed differently than how AMD does
it natively, they are exposed to Windows as sockets, which means, that
if you are nto a business user, you can't ever have a machine with
more than two CCX (6 cores) as consumer versions of Windows only
supports two sockets. (Should this be reported as a separate bug?)
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