[Bug 1988527] Re: autopkgtest_qemu doesn't use accel=kvm on ppc64le, being fully unusable on that arch

[Christian Ehrhardt ]

TL;DR:
- either use "$ kvm" instead of "$ qemu-system-*"
- or (better) use accel=... according to your preferences
  - I assume you want kvm:tcg to try the fast but not break if falling back
- doing neither of the above most likely will give you only TCG

Details follow ...

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Title:
  autopkgtest_qemu doesn't use accel=kvm on ppc64le, being fully
  unusable on that arch

Status in autopkgtest package in Ubuntu:
  New

Bug description:
  On Power9 the qemu based autopkgtest commands create VMs that are
  extremely slow and fail with obscure errors (partially discussed in
  LP: #1973628, comment 8). This can be reproduced for example by
  running:

    autopkgtest-buildvm-ubuntu-cloud -v -r jammy --ram-size 1024

  but autopkgtest-virt-qemu is also affected. The extreme slowness of
  the VMs made me think that something was off with the virtualization
  settings. I modified autopkgtest_qemu.py so that qemu-system-ppc64le
  is called with '-machine accel=kvm' (which I think is the same as
  '-machine pseries,accel=kvm' with pseries being the default machine
  type).

  With this change everything is very fast and reliable. These warnings
  also went away:

  qemu-system-ppc64le: warning: TCG doesn't support requested feature, cap-cfpc=workaround
  qemu-system-ppc64le: warning: TCG doesn't support requested feature, cap-sbbc=workaround
  qemu-system-ppc64le: warning: TCG doesn't support requested feature, cap-ibs=workaround
  qemu-system-ppc64le: warning: TCG doesn't support requested feature, cap-ccf-assist=on

  indicating that we were using TCG emulation before.

  I imagine that Qemu has good reasons not to default to accel=kvm or
  accel=kvm:tcg on ppc64, but think it's reasonable to assume it's
  available and enable it in autopkgtest.

  We can fix this in autopkgtest upstream, but it would be nice to
  verify if this is an issue with Debian too before submitting a salsa
  MR.

  [1] https://wiki.qemu.org/Documentation/TCG

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