[libvirt PATCH] meson: Check usability of linux/kvm.h

[Andrea Bolognani]

On Thu, Apr 27, 2023 at 11:28:36AM +0100, Daniel P. Berrangé wrote:
> On Thu, Apr 27, 2023 at 12:00:59PM +0200, Andrea Bolognani wrote:
> > On Thu, Apr 27, 2023 at 11:50:05AM +0200, Michal Prívozník wrote:
> > > But leaving meson aside, shouldn't we just use check_header() for every
> > > header file then? I mean, this fixes this particular instance of the
> > > problem, but can't we hit it again with say ifaddrs.h or any other
> > > header file on the list?
> >
> > We could. The Meson documentation recommends not doing this, as
> > check_header() is (understandably) slower than has_header().
> >
> > I haven't tried to see how much slower we're actually talking...
> > Maybe for the 20-ish headers that we care about, it wouldn't make a
> > measurable difference? Especially since the results of the check are
> > cached between Meson runs.
>
> I tried testing:
>
> diff --git a/meson.build b/meson.build
> index c15003ce02..3ae02ce4df 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -633,14 +633,14 @@ if host_machine.system() == 'freebsd'
>  endif
>
>  foreach name : headers
> -  if cc.has_header(name)
> +  if cc.check_header(name)
>      conf.set('WITH_ at 0@'.format(name.underscorify().to_upper()), 1)
>    endif
>  endforeach
>
>  # check for kernel header required by src/util/virnetdevbridge.c
>  if host_machine.system() == 'linux'
> -  if not cc.has_header('linux/sockios.h')
> +  if not cc.check_header('linux/sockios.h')
>      error('You must install kernel-headers in order to compile libvirt with QEMU or LXC support')
>    endif
>  endif
>
>
> and could not measure any deterministic change in execution
> time before/after

I've just tried the same on a 4th generation low-voltage laptop CPU,
with a patch that goes further and completely replaces all uses of
has_header() with check_header(), and haven't been able to detect any
meaningful change either. Let's go with the simpler and more reliable
approach then :)

-- 
Andrea Bolognani / Red Hat / Virtualization