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

[Eric Blake]

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).

Yes, that would be a bigger patch, but it may also be easier to
maintain down the road.  And even though there is an #ifdef ladder at
runtime, it only has O(n) growth rather than O(n^2) for each possible
combination of which options are enabled, and would appear only once
in the runtime rather than duplicated across each place which
documents similar help text across multiple utilities.

> 
> > 
> > Reviewed-by: Eric Blake <eblake@redhat.com>
> > Signed-off-by: Dirk Müller <dmueller@suse.de>
> > ---
> >  block/file-posix.c | 4 ++++
> >  qemu-nbd.c         | 4 ++++
> >  qemu-options.hx    | 6 +++++-
> >  3 files changed, 13 insertions(+), 1 deletion(-)
> > 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org