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Re: [RFC PATCH 0/5] hw/arm/virt: Introduce cpu topology support

From: Ying Fang
Subject: Re: [RFC PATCH 0/5] hw/arm/virt: Introduce cpu topology support
Date: Fri, 26 Feb 2021 16:41:45 +0800
User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0

On 2/25/2021 8:02 PM, Andrew Jones wrote:
On Thu, Feb 25, 2021 at 04:56:22PM +0800, Ying Fang wrote:
An accurate cpu topology may help improve the cpu scheduler's decision
making when dealing with multi-core system. So cpu topology description
is helpful to provide guest with the right view. Dario Faggioli's talk
in [0] also shows the virtual topology may has impact on sched performace.
Thus this patch series is posted to introduce cpu topology support for
arm platform.

Both fdt and ACPI are introduced to present the cpu topology. To describe
the cpu topology via ACPI, a PPTT table is introduced according to the
processor hierarchy node structure. This series is derived from [1], in
[1] we are trying to bring both cpu and cache topology support for arm
platform, but there is still some issues to solve to support the cache
hierarchy. So we split the cpu topology part out and send it seperately.
The patch series to support cache hierarchy will be send later since
Salil Mehta's cpu hotplug feature need the cpu topology enabled first and
he is waiting for it to be upstreamed.

This patch series was initially based on the patches posted by Andrew Jones [2].
I jumped in on it since some OS vendor cooperative partner are eager for it.
Thanks for Andrew's contribution.

After applying this patch series, launch a guest with virt-6.0 and cpu
topology configured with sockets:cores:threads = 2:4:2, you will get the
bellow messages with the lscpu command.

Architecture:                    aarch64
CPU op-mode(s):                  64-bit
Byte Order:                      Little Endian
CPU(s):                          16
On-line CPU(s) list:             0-15
Thread(s) per core:              2

What CPU model was used? Did it actually support threads? If these were

It's tested on Huawei Kunpeng 920 CPU model and vcpu host-passthrough.
It does not support threads for now, but the next version 930 may
support it. Here we emulate a virtual cpu topology, a virtual 2 threads
is used to do the test.

KVM VCPUs, then I guess MPIDR.MT was not set on the CPUs. Apparently
that didn't confuse Linux? See [1] for how I once tried to deal with


If ACPI PPTT table is specified, the linux kernel won't check the MPIDR
register to populate cpu topology. Moreover MPIDR does not ensure a
right cpu topology. So it won't be a problem if MPIDR.MT is not set.

Core(s) per socket:              4
Socket(s):                       2

Good, but what happens if you specify '-smp 16'? Do you get 16 sockets
each with 1 core? Or, do you get 1 socket with 16 cores? And, which do
we want and why? If you look at [2], then you'll see I was assuming we
want to prefer cores over sockets, since without topology descriptions
that's what the Linux guest kernel would do.


NUMA node(s):                    2

Why do we have two NUMA nodes in the guest? The two sockets in the
guest should not imply this.

The two NUMA nodes are emulated by Qemu since we already have guest numa
topology feature. So the two sockets in the guest has nothing to do with
it. Actually even one socket may have two numa nodes in it in real cpu


Vendor ID:                       HiSilicon
Model:                           0
Model name:                      Kunpeng-920
Stepping:                        0x1
BogoMIPS:                        200.00
NUMA node0 CPU(s):               0-7
NUMA node1 CPU(s):               8-15

[1] https://lists.gnu.org/archive/html/qemu-devel/2020-11/msg02166.html

Ying Fang (5):
   device_tree: Add qemu_fdt_add_path
   hw/arm/virt: Add cpu-map to device tree
   hw/arm/virt-acpi-build: distinguish possible and present cpus
   hw/acpi/aml-build: add processor hierarchy node structure
   hw/arm/virt-acpi-build: add PPTT table

  hw/acpi/aml-build.c          | 40 ++++++++++++++++++++++
  hw/arm/virt-acpi-build.c     | 64 +++++++++++++++++++++++++++++++++---
  hw/arm/virt.c                | 40 +++++++++++++++++++++-
  include/hw/acpi/acpi-defs.h  | 13 ++++++++
  include/hw/acpi/aml-build.h  |  7 ++++
  include/hw/arm/virt.h        |  1 +
  include/sysemu/device_tree.h |  1 +
  softmmu/device_tree.c        | 45 +++++++++++++++++++++++--
  8 files changed, 204 insertions(+), 7 deletions(-)



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