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Re: Conflicting parameters on qemu call



On 21. Aug 2020, at 11:20, Daniel P. Berrangé <berrange redhat com> wrote:
On Fri, Aug 21, 2020 at 11:19:14AM +0200, Jan Walzer wrote:
Hi Daniel,
On 21. Aug 2020, at 11:07, Daniel P. Berrangé <berrange redhat com> wrote:
On Fri, Aug 21, 2020 at 11:00:27AM +0200, Jan Walzer wrote:
On 21. Aug 2020, at 10:38, Daniel P. Berrangé <berrange redhat com> wrote:
On Thu, Aug 20, 2020 at 08:11:30PM +0200, Jan Walzer wrote:
Hi Lists,

I currently have the issue of wanting to use emu-system-x86_64 on a ppc64le platform.

It is imperative to pass the "-accel tcg,thread=multi” parameter to qemu 
when starting an instance, as without that, it will only use one thread 
and hence of limited/no use.

The problem is, that libvirt itself, passes “-machine q35,accel=tcg” to
qemu, which is a different parameter, that conflicts with the other one.

Can we discuss, if I either have overlooked something, or is there a workaround, or is this a bug?

What you're trying todo is intentionally not available.

The memory ordering constraints needed for running x86_64 guests on ppc64
hosts cannot be satisfied, so multi-threaded TCG is not available.

For any guest/host combination where multi-thread TCG is safe to use, QEMU
will enable it automatically, so nothing is required in libvirt.


Hi Daniel,
Thanks for the answer. I’ve read (and understand) the warnings and their implications.

So there’s not even an “I know what I’m Doing”-Switch?

[…]
<qemu:commandline>
  <qemu:arg value='-accel'/>
  <qemu:arg value='tcg,thread=multi'/>
</qemu:commandline>

As I wrote: This is what I’m already doing
The Problem is that this conflicts with, libvirt already using the parameter: “-machine ..,accel=tcg”
And I can’t get libvirt, to stop passing this parameter.

It shouldn't matter.  QEMU should aggregate the settings from the
multiple args, with latter args overriding earlier args if there is
any duplication..

That was what I thought to, but:

# qemu-system-x86_64  -machine q35,accel=tcg -accel tcg,thread=multi --smp 8 -m 8
Unable to init server: Could not connect: Connection refused
qemu-system-x86_64: The -accel and "-machine accel=" options are incompatible

# qemu-system-x86_64 --version
QEMU emulator version 5.0.0 (Debian 1:5.0-14)
Copyright (c) 2003-2020 Fabrice Bellard and the QEMU Project developers

I’m already discussing that on qemu list:
https://lists.nongnu.org/archive/html/qemu-ppc/2020-08/msg00338.html

But I’d like to track that issue in multiple directions.

Greetings, Jan


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