Re: [PATCH v2] Only advertise aio=io_uring if support is actually available

[Daniel P . Berrangé]

On Fri, Apr 22, 2022 at 03:30:46PM -0500, Eric Blake wrote:
> On Thu, Apr 21, 2022 at 02:27:55PM -0500, Eric Blake wrote:
> > On Thu, Apr 21, 2022 at 06:50:48PM +0200, Dirk Müller wrote:
> > > Change --help output for aio option to only list the aio backend options 
> > > that
> > > are actually available. io_uring is an optional, linux only backend
> > > option so hide it for cases where it isn't there.
> > 
> > As pointed out by Dan, this commit message is not quite accurate.  It
> > hides only one of the two conditional options, but 'native' is also a
> > Linux-only optional backend (CONFIG_LINUX_AIO).
> 
> Stepping back a bit - we already said that making --help
> machine-parseable is a non-goal, and so the intent of this patch is
> for human readers.  But adding an #ifdef ladder to show all 4 possible
> combinations gets hairy; adding another option turns it into 8
> combinations.  Is there a better way
> 
> What if we just document ALL strings possible in at least one build
> (no #ifdef ladder in the help text), but add --aio=help as a way to do
> a runtime list of which aio modes are understood by THIS build?  It
> would match how we have --device=help for qemu proper, and may even be
> able to reuse some of that framework code (for parsing out when help
> is requested).

Did you literally mean '--aio=help' ?  'aio' is just an option
for disks, and I don't think we want new top levelk options. So
would have to be '-drive ....,aio=help', or '--blockdev ....aio=help'
the latter gets tricky with nested json syntax though, so not sure
it is nicer.

With regards,
Daniel
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