Re: make vm-build-freebsd appears to require . in PATH

[Markus Armbruster <armbru@redhat.com>]

Peter Maydell <peter.maydell@linaro.org> writes:

> On Tue, 10 Jan 2023 at 16:26, Markus Armbruster <armbru@redhat.com> wrote:
>> Peter Maydell <peter.maydell@linaro.org> writes:
>> > Does it actually require '.' on the PATH, or does it just want
>> > a qemu-img binary on the PATH? (eg your distro one in /usr/bin).
>> > I don't have '.' on my PATH and it works for me.
>>
>> Do we want to use qemu-img, qemu-system-x86_64 and so forth from PATH,
>> or the one in the build tree?
>
> There's no guarantee there is one in the build tree at all.
> I usually use these like
>  (cd build && ../configure)
>  make -C build  vm-build-openbsd
>
> in which case it doesn't need to build anything in the build
> tree at all (neither qemu-system-x86_64 nor qemu-img).
>
> It's nice to be able to do "test this build on *BSD" with
> a known-good QEMU running the VM rather than having the
> code-under-test affecting both the outer QEMU and the
> build-and-make-check running inside the VM.

True.

>> The former could well be old, which feels like a potential source of
>> problems.
>
> In practice we only use it for very simple operations
> ("create a qcow2 image" and "resize this qcow2 file"),
> so using the distro qemu-img has never been an issue for me.
>
> I think I have in the past run into problems because the
> system's qemu-system-x86_64 was super-old, but it was easy
> to just build a known-good QEMU version and put that on
> the PATH. (And now that system has had a host distro
> upgrade, so I have gone back to using the system binary.)

I since came to understand this line in output of vm-help:

    QEMU_LOCAL=1                 - Use QEMU binary local to this build.

So the intent appears to be "use (presumably known-good) QEMU tooling
from $PATH by default, pass QEMU_LOCAL=1 to use the build tree instead,
and pass QEMU=... QEMU_IMG=... QEMU_CONFIG=... when you need even more
control."

Thanks again!