Re: [Qemu-devel] [PATCH] Add configure clock_gettime() monotonic time test

[Paolo Bonzini <pbonzini@redhat.com>]

> On Wed, Apr 03, 2013 at 10:18:51AM +0200, Paolo Bonzini wrote:
> > Il 03/04/2013 00:52, Peter Maydell ha scritto:
> > > On 2 April 2013 17:45, Paolo Bonzini <pbonzini@redhat.com> wrote:
> > >> Il 02/04/2013 17:42, Brad Smith ha scritto:
> > >>> Replace the hardcoded list of OS's utilizing clock_gettime() for
> > >>> monotonic
> > >>> time with a configure test. This is to fix the use of monotonic time on
> > >>> OpenBSD but allows for other POSIX compliant OS's such as NetBSD to
> > >>> also
> > >>> utilize clock_gettime().
> > >>
> > >> I thought the list of OSes was supposed to filter out those that somehow
> > >> had a broken CLOCK_MONOTONIC.
> > > 
> > > Judging from wading through git history, it's mostly just
> > > historic accretion from an initial #ifdef __linux__ which was
> > > put in by Fabrice way back when configure was barely doing
> > > compile-this-code checks at all.
> > > 
> > > Google does suggest that some OSes do provide a CLOCK_MONOTONIC
> > > but clock_gettime(CLOCK_MONOTONIC) always fails, eg Centos 3.
> > 
> > It would already fail, and no one reported it.
> > 
> > CentOS 3 has a 2.4 kernel.  I doubt anyone is using it with a recent QEMU.
> > 
> > Paolo
> > 
> > >> Otherwise, you might as well use "#ifdef CLOCK_MONOTONIC" and skip the
> > >> configure test completely.
> > > 
> > > Tempting.
> > > 
> > > -- PMM
> 
> Then how about the following? This looks like it should be good
> for Linux, *BSD's, Solaris.
> 
> 
> diff --git a/include/qemu/timer.h b/include/qemu/timer.h
> index 1766b2d..c363190 100644
> --- a/include/qemu/timer.h
> +++ b/include/qemu/timer.h
> @@ -117,8 +117,7 @@ extern int use_rt_clock;
>  
>  static inline int64_t get_clock(void)
>  {
> -#if defined(__linux__) || (defined(__FreeBSD__) && __FreeBSD_version >=
> 500000) \
> -    || defined(__DragonFly__) || defined(__FreeBSD_kernel__)
> +#ifdef CLOCK_MONOTONIC
>      if (use_rt_clock) {
>          struct timespec ts;
>          clock_gettime(CLOCK_MONOTONIC, &ts);
> diff --git a/util/qemu-timer-common.c b/util/qemu-timer-common.c
> index 16f5e75..95e0847 100644
> --- a/util/qemu-timer-common.c
> +++ b/util/qemu-timer-common.c
> @@ -49,9 +49,7 @@ int use_rt_clock;
>  static void __attribute__((constructor)) init_get_clock(void)
>  {
>      use_rt_clock = 0;
> -#if defined(__linux__) || (defined(__FreeBSD__) && __FreeBSD_version >=
> 500000) \
> -    || defined(__DragonFly__) || defined(__FreeBSD_kernel__) \
> -    || defined(__OpenBSD__)
> +#ifdef CLOCK_MONOTONIC
>      {
>          struct timespec ts;
>          if (clock_gettime(CLOCK_MONOTONIC, &ts) == 0) {
> 

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

Paolo