Re: [Qemu-devel] [PATCH] Add compat eventfd header

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jun 29, 2011 at 03:22:33PM +0200, Alexander Graf wrote:
> 
> On 29.06.2011, at 15:11, Michael S. Tsirkin wrote:
> 
> > On Wed, Jun 29, 2011 at 03:02:46PM +0200, Alexander Graf wrote:
> >> 
> >> On 28.06.2011, at 17:35, Michael S. Tsirkin wrote:
> >> 
> >>> Support build on RHEL 5.X where we have syscall for eventfd but not
> >>> userspace wrapper.
> >>> 
> >>> (cherry-picked from commit 9e3269181e9bc56feb43bcd4e8ce0b82cd543e65
> >>> in qemu-kvm.git).
> >>> 
> >>> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> >>> ---
> >>> compat/sys/eventfd.h |   13 +++++++++++++
> >>> configure            |    4 +++
> >>> 2 files changed, 16 insertions(+), 0 deletions(-)
> >>> create mode 100644 compat/sys/eventfd.h
> >>> 
> >>> diff --git a/compat/sys/eventfd.h b/compat/sys/eventfd.h
> >>> new file mode 100644
> >>> index 0000000..f55d96a
> >>> --- /dev/null
> >>> +++ b/compat/sys/eventfd.h
> >>> @@ -0,0 +1,13 @@
> >>> +#ifndef _COMPAT_SYS_EVENTFD
> >>> +#define _COMPAT_SYS_EVENTFD
> >>> +
> >>> +#include <unistd.h>
> >>> +#include <syscall.h>
> >>> +
> >>> +
> >>> +static inline int eventfd (int count, int flags)
> >> 
> >> coding style seems wrong.
> > 
> > What exactly? Two empty lines?
> 
> The space between d and ( I'd say. Just put it in checkpatch and verify it :).

Will fix.

> > 
> >> However, I'm not sure I like the idea of adding this code in qemu. Wouldn't the RHEL5 libc be a better place for such a wrapper?
> >> 
> >> 
> >> Alex
> > 
> > My guess (I don't speak for red hat here) is that's unlikely to be
> > patched anytime soon.  It helps me when I need to use such a box,
> > and the cost seems negligeable. What's the drawback?
> 
> Well, you need to make sure that it only gets included on Linux systems and if there's ever some more compatibility wrapping around the syscall (unlikely, but you never know), this could potentially break.

Nope, this gets included last (-idirafter) so if it breaks it's broken
anyway.

> Also, who defines SYS_eventfd? What if you're trying to build this code on SLES10 for example, which does not have the syscall and thus doesn't have it defined? Would compilation simply break?
> 
> 
> Alex

Here's what happens:
1. configure runs
2. configure tries to compile a test program
	- if eventfd.h exists in system compat is ignored
	- if eventfd.h does not exist in system compat is used
	- if compat is used but does build program does not compile
	- if program does not compile eventfd is disabled

3. now build starts
	- if we had trouble compiling stuff eventfd gets disabled

I think the setup is highly safe.

-- 
MST