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

[Daniel P. Berrangé]

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:
> > On 4/27/23 11:41, Andrea Bolognani wrote:
> > > +# headers for which we need to check actual usability. in most
> > > +# cases, checking for presence is enough (and it's way faster)
> > > +check_headers = [
> > > +  'linux/kvm.h',
> > > +]
> > > +
> > > +foreach name : check_headers
> > > +  if cc.check_header(name)
> > > +    conf.set('WITH_ at 0@'.format(name.underscorify().to_upper()), 1)
> > > +  endif
> > > +endforeach
> >
> > One could argue, that the semantics of has_header() is broken then. What
> > good is there to check if the host has a header file when compiler
> > rejects it subsequently?
> >
> > 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


With regards,
Daniel
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