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Re: Conflicting parameters on qemu call



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?

You could use command line passthrough

  https://libvirt.org/drvqemu.html#qemucommand

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

This marks the guest tainted as we consider command line passthrough
unsupported, but your use of multi-threaded TCG in QEMU is already
unsupported, so no worse.

> I have verified my use case with the manual calling of qemu with 
> the mentioned parameters and my test suite for that use case works 
> fine. 

It may appear to work fine today but don't be surprised if it fails
at any time. The kind of problems exposed by this scenario can be
very subtle and only hit if you tickle particular races. So it may
work 49 times in 50, and in the other 1 case silently correct your
data.

Regards,
Daniel
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