Re: [Qemu-devel] [PATCH] hw/arm: set machine 'virt' as default

[Dan Streetman]

On Wed, Sep 18, 2019 at 4:34 PM Alex Bennée <address@hidden> wrote:
>
>
> Dan Streetman <address@hidden> writes:
>
> > From: Dan Streetman <address@hidden>
> >
> > There is currently no default machine type for arm so one must be specified
> > with --machine.  This sets the 'virt' machine type as default.
>
> We should really have a FAQ entry for why we don't have a default for
> ARM. In short unlike PC's every ARM device is different so it pays to be
> precise about what you want when you invoke QEMU. Because any given
> kernel/image is only likely to work on the machine it's built for.

well, that's the problem, I have no idea at all what I want; and "I"
doesn't really apply completely in this situation, as the call to run
qemu comes from deep inside a test suite, and can run on multiple
archs, and could even be run by other people on other systems/archs.

This is what I have (tentatively) come up with to handle this in the test suite:
https://github.com/systemd/systemd/pull/13409/files#diff-2ea30ffea3b108e0f9c50846cfdcd4e5R197

To be fair, it's unlikely that other people would run this on an arm
system, unless they were a bit more familiar with arm, and maybe would
know what machine type to pick.  Similarly for the testbeds that I
handle for this test suite, I know that 'virt' seems to work.

>
> Why is virt special? It's just one of the many machines we emulate and
> while it's probably the most popular these days for "something that
> boots a Linux distro" why not -machine sba (when that comes)?

I am certainly not the right person to pick what the default should
be, but I do think there should be *some* default.  If 'virt' is the
most popular and/or has the widest kernel support, then it probably
makes sense to make that the default.

I would guess that users of qemu-system-aarch64 (or -arm) fall into 2 groups:

1. people who know about arm and know exactly what machine they want to use
2. people who don't know about arm and have no idea what machine to use

group #1 of course can still pick whatever machine they want.  I'm in
group #2, and I suspect that like most others in the group, I did:

$ qemu-system-aarch64 ...
qemu-system-aarch64: No machine specified, and there is no default
Use -machine help to list supported machines
$ qemu-system-aarch64 -M ?
...shows long list of machines that i'm unfamiliar with...
virt-2.10            QEMU 2.10 ARM Virtual Machine
virt-2.11            QEMU 2.11 ARM Virtual Machine
virt-2.12            QEMU 2.12 ARM Virtual Machine
virt-2.6             QEMU 2.6 ARM Virtual Machine
virt-2.7             QEMU 2.7 ARM Virtual Machine
virt-2.8             QEMU 2.8 ARM Virtual Machine
virt-2.9             QEMU 2.9 ARM Virtual Machine
virt-3.0             QEMU 3.0 ARM Virtual Machine
virt                 QEMU 3.1 ARM Virtual Machine (alias of virt-3.1)
virt-3.1             QEMU 3.1 ARM Virtual Machine

(aha! those "virt" machines look generic enough that they'll work...)
$ qemu-system-aarch64 -M virt ...

I honestly don't know if it would be better to have a FAQ on why there
is no default, or just to set a default.  Personally, I'd prefer just
having a default.

If you do decide against a default, I would suggest at least printing
the url to the FAQ entry on why arm doesn't have a default, instead of
just asking users to pick one out of the -M ? list.


>
> >
> > Signed-off-by: Dan Streetman <address@hidden>
> > ---
> >  hw/arm/virt.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> > index d74538b021..e9fe888ca2 100644
> > --- a/hw/arm/virt.c
> > +++ b/hw/arm/virt.c
> > @@ -78,6 +78,7 @@
> >          mc->desc = "QEMU " # major "." # minor " ARM Virtual Machine"; \
> >          if (latest) { \
> >              mc->alias = "virt"; \
> > +            mc->is_default = 1; \
> >          } \
> >      } \
> >      static const TypeInfo machvirt_##major##_##minor##_info = { \
>
>
> --
> Alex Bennée
>