Re: [Qemu-devel] [PATCH 0/3] -readconfig: accept fd=<fd> option (v2)

[Eduardo Habkost]

On Mon, Mar 19, 2012 at 11:21:07AM -0500, Anthony Liguori wrote:
> On 03/19/2012 11:06 AM, Eduardo Habkost wrote:
> >On Mon, Mar 19, 2012 at 10:39:10AM -0500, Anthony Liguori wrote:
> >>On 03/19/2012 10:37 AM, Eduardo Habkost wrote:
> >>>On Mon, Mar 19, 2012 at 10:10:27AM -0500, Anthony Liguori wrote:
> >>>>On 03/19/2012 09:54 AM, Eduardo Habkost wrote:
> >>>>>This is a resubmit of a previous series I sent as a RFC, with some 
> >>>>>changes to
> >>>>>prepare for an upcoming patch that will make additional changes to the 
> >>>>>default
> >>>>>config-file loading code.
> >>>>>
> >>>>>This series needs be applied on top of the "./configure --confdir" 
> >>>>>series I
> >>>>>sent today.
> >>>>
> >>>>Why not just use /dev/fd/N  ?
> >>>
> >>>Personally, I don't like filenames with special meanings (as not every
> >>>OS has /dev/fd we would have to treat them specially), or filenames that
> >>>become non-extensible mini-languages by themselves. Many other
> >>>command-line options use the key=value syntax, and some already have an
> >>>"fd" option, so this just follows the convention.
> >>
> >>But you're also breaking compat, which is not something to be done lightly.
> >
> >True. In that case, I propose adding a "-config" option that is more
> >extensible than the current one, instead of defining a new mini-language
> >for -readconfig that looks like a file path but has hidden complexities
> >behind it.
> 
> What?  /dev/fd is a pretty standard unix feature.  It'll work today
> with existing versions of QEMU.

When I replied, I was thinking of this proposal:
http://marc.info/?l=qemu-devel&m=133076148831720
If you are proposing having zero special-case code to Qemu because of
/dev/fd, it's OK to me.

> >>>Also, this is more extensible to allow more options to be added to
> >>>-readconfig if needed (for example: debugging options, or the
> >>>help=defconfig option I added on the RFC series I sent after this one).
> >>
> >>I'd personally prefer to keep readconfig simple.  See the series I
> >>sent out as an RFC.
> >
> >If you are supporting FDs, the complexity is already there. Using the
> >key=value format just makes the complexity explicit (and familiar, for
> >humans and code that already uses key=value for other options) instead
> >of hiding it behind magic filenames.
> >
> >I still have to take a look at your series.
> 
> I think there are cases where /dev/fd doesn't make any sense as an
> interface (like tun/tap) because tun/tap doesn't have normal file
> semantics.
> 
> Likewise, block devices don't act like normal files so you need to
> treat fd differently than you treat a file path.
> 
> But for something like the BIOS file, config files, kernels, etc,
> they are all just simple files and instead of making everything take
> a fd:, we can (and should) just use the existing unix mechanism for
> this.

OK to me, if that doesn't require code to handle "/dev/fd/" as special
inside Qemu, but just use what the OS provides. I am just against making
magic filenames special.

I was still considering adding a key=value interface for -readconfig (or
an equivalent -config parameter) because of the debugging and probing
options I would like to add, but those options could be enabled/disabled
somewhere else, so there wouldn't be a strong reason for it anymore.

On the other hand, I:

- Would like to let the user (a human) list where are the default config
  files being used by Qemu;
- Don't want to add Yet Another "-option ?" parameter to Qemu (e.g.
  "-readconfig ?").

Do you have any suggestions for that?

-- 
Eduardo