Re: [Qemu-devel] [PATCH] Support 'help' as a synonym for '?' in command line options

[Daniel P. Berrange]

On Mon, Jul 09, 2012 at 06:07:48AM -0600, Eric Blake wrote:
> On 07/09/2012 05:52 AM, Peter Maydell wrote:
> > For command line options which permit '?' meaning 'please list the
> > permitted values', add support for 'help' as a synonym, by abstracting
> > the check out into a helper function.
> > 
> > Update the documentation to use 'help' rather than '?', since '?'
> > is a shell metacharacter and thus prone to fail confusingly if there
> > is a single character filename in the current working directory and
> > the '?' has not been escaped. It's therefore better to steer users
> > towards 'help', though '?' is retained for backwards compatibility.
> 
> I like the idea, but this may cause grief for libvirt, which basically does:
> 
> const char *help; // = output of 'qemu -h'
> if (strstr(help, "-device driver,?")) {
>     /* Cram together all device-related queries into one invocation;
>      * the output format makes it possible to distinguish what we
>      * need.  With qemu 0.13.0 and later, unrecognized '-device
>      * bogus,?' cause an error in isolation, but are silently ignored
>      * in combination with '-device ?'.  Upstream qemu 0.12.x doesn't
>      * understand '-device name,?', and always exits with status 1 for
>      * the simpler '-device ?', so this function is really only useful
>      * if -help includes "device driver,?".  */
>     cmd = virCommandNewArgList(qemu,
>                                "-device", "?",
>                                "-device", "pci-assign,?",
>                                "-device", "virtio-blk-pci,?",
>                                "-device", "virtio-net-pci,?",
>                                "-device", "scsi-disk,?",
>                                NULL);
> }
> 
> That is, we are filtering based on the explicit presence of a literal
> '?' in the help output to determine whether we can further filter based
> on '-device device,?' queries without confusing qemu or libvirt;
> changing the 'help' output means that old libvirt with new qemu won't
> run the extra queries (as if it had been targeting qemu 0.12.x), even
> though the older spelling of the query would still work.
> 
> If that is not a concern (that is, if you are willing to state that use
> of newer qemu is intended to be coupled with newer libvirt that has been
> taught to use 'help' instead of '?'), then this patch is probably fine.
>  The alternative is to update 'qemu -h' output to mention both forms,
> rather than completely eradicating the ? form from documentation.


Regardless of what QEMU does here, I think we should change libvirt's
handling of this arg. I think we can probably just skip that check, and
run 'qemu -device XXX,?' unconditionally and then handle any error that
we get back from old QEMU.

Regards,
Daniel
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