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Re: Priority of -accel (was: [PATCH] tests/qemu-iotests: Update tests to

From: Peter Maydell
Subject: Re: Priority of -accel (was: [PATCH] tests/qemu-iotests: Update tests to recent desugarized -accel option)
Date: Tue, 7 Jan 2020 17:53:20 +0000

On Tue, 7 Jan 2020 at 17:44, Christophe de Dinechin <address@hidden> wrote:
> > On 7 Jan 2020, at 15:37, Paolo Bonzini <address@hidden> wrote:
> > It would break backwards compatibility for "-machine accel=tcg:kvm",
> > which so far meant "use TCG if compiled in, otherwise use KVM".  This is
> > not something I would have a problem with... except that "tcg:kvm" is
> > the default if no -accel option is provided!
> What is the rationale for picking tcg over kvm?

Command line compatibility. QEMU existed before KVM support
was ever merged into it, and so the default has always been
"emulate the CPU". Defaulting to KVM would be a change of
behaviour for existing working command lines. The reason
for using "tcg:kvm" rather than just "tcg" may be that
this way a --disable-tcg QEMU does something useful, but that's
just a guess.

QEMU tends to prioritize "don't change behaviour for users"
to a degree that is arguably counterproductive. But it's a
nice easy rule to follow :-)

-- PMM

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