[libvirt] [Qemu-devel] [PATCH v3 2/2] Add configure option --enable-pc-1-0-qemu-kvm

[Markus Armbruster]

"Michael S. Tsirkin" <mst at redhat.com> writes:

> On Mon, Sep 22, 2014 at 05:32:16PM +0200, Markus Armbruster wrote:
>> "Michael S. Tsirkin" <mst at redhat.com> writes:
>> 
>> > On Sun, Sep 21, 2014 at 03:38:59PM +0100, Alex Bligh wrote:
>> >> Add a configure option --enable-pc-1-0-qemu-kvm and the
>> >> corresponding --disable-pc-1-0-qemu-kvm, defaulting
>> >> to disabled.
>> >> 
>> >> Rename machine type pc-1.0 to pc-1.0-qemu-git.
>> >> 
>> >> Make pc-1.0 machine type an alias of either pc-1.0-qemu-kvm
>> >> or pc-1.0-qemu-git depending on the value of the config
>> >> option.
>> >> 
>> >> Signed-off-by: Alex Bligh <alex at alex.org.uk>
>> >
>> > I have to say, this one bothers me.
>> > We end up not being able to predict what does pc-1.0
>> > reference.
>> >
>> > Users also don't get qemu from git so I don't see
>> > why does git make sense in the name?
>> >
>> > Legacy management applications invoked qemu as qemu-kvm -
>> > how about detecting that name and switching
>> > the machine types?
>> 
>> Ugh!  I like that even less than a configure option.
>
> configure options are tested even less than runtime ones:
> each distro has to pick up a set of options and all
> users are stuck with it.
> So let's assume Ubuntu sets this and something is broken.  How is a
> Fedora user to test this? Find out configure flags that Ubuntu uses
> and rebuild from source? It's hard to get people to triage bugs as it
> is.
> That's why changing behaviour in subtle ways depending on
> configure options is a very bad idea.

All it changes is a default machine type.  I wouldn't classify that as
"very bad".  Fortunately, our difference of opinion doesn't matter,
because the conclusion of the thread is "leave it to the distros".

> So a flag might be better, but if we want to be compatible
> with qemu-kvm, poking at argv[0] still better than configure.

That I do consider a genuinely bad idea.

Example of usage messed up by it: I symlink from my ~/bin to executables
in various build trees.  If one of these programs changed behavior
depending on what it finds in argv[0], I'd have to know *exactly* how it
matches argv[0] to choose suitable names for my symlinks.

The name "qemu-kvm" is not a useful indicator of compatibility with the
qemu-kvm fork of QEMU anyway.  There are programs out there calling
themselves qemu-kvm that aren't compatible with the qemu-kvm fork.  And
there are programs out there that are compatible, but call themselves
something else.