Re: [PATCH RFC] qga: fence guest-set-time if hwclock not available

[Cornelia Huck <cohuck@redhat.com>]

On Thu, 28 Nov 2019 12:45:32 +0000
Daniel P. Berrangé <berrange@redhat.com> wrote:

> On Thu, Nov 28, 2019 at 01:36:58PM +0100, Cornelia Huck wrote:
> > The Posix implementation of guest-set-time invokes hwclock to
> > set/retrieve the time to/from the hardware clock. If hwclock
> > is not available, the user is currently informed that "hwclock
> > failed to set hardware clock to system time", which is quite
> > misleading. This may happen e.g. on s390x, which has a different
> > timekeeping concept anyway.
> > 
> > Let's check for the availability of the hwclock command and
> > return QERR_UNSUPPORTED for guest-set-time if it is not available.
> > 
> > Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> > ---
> > 
> > Not sure if that is the correct approach, but the current error
> > message is really quite confusing.  
> 
> I guess the alternative is to just #ifndef __s390x__ the whole
> impl of the qmp_guest_set_time  method, but I don't have a
> strong opinion on which is best.

This hardcodes this as a s390x specialty, though, and I'm not sure that
assumption is and will stay correct.

> 
> > 
> > Gave it a quick test with an s390x and an x86_64 guest; invoking
> > 'virsh domtime <value>' now fails with 'not currently supported'
> > on s390x and continues to work as before on x86_64.
> > 
> > ---
> >  qga/commands-posix.c | 20 +++++++++++++++++++-
> >  1 file changed, 19 insertions(+), 1 deletion(-)
> > 
> > diff --git a/qga/commands-posix.c b/qga/commands-posix.c
> > index 1c1a165daed8..bd298a38b716 100644
> > --- a/qga/commands-posix.c
> > +++ b/qga/commands-posix.c
> > @@ -149,6 +149,13 @@ int64_t qmp_guest_get_time(Error **errp)
> >     return tq.tv_sec * 1000000000LL + tq.tv_usec * 1000;
> >  }
> >  
> > +static int check_hwclock_available(const char *path)
> > +{
> > +    struct stat st;
> > +
> > +    return (stat(path, &st) < 0) ? 0 : 1;
> > +}
> > +
> >  void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
> >  {
> >      int ret;
> > @@ -156,6 +163,17 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
> >      pid_t pid;
> >      Error *local_err = NULL;
> >      struct timeval tv;
> > +    const char *hwclock_path = "/sbin/hwclock";
> > +    static int hwclock_available = -1;
> > +
> > +    if (hwclock_available < 0) {
> > +        hwclock_available = check_hwclock_available(hwclock_path);  
> 
> Could do this inline with:
> 
>     hwclock_available = (access(hwclock_available, X_OK) == 0);
> 
> getting a slightly better result as this check for it being
> executable as well as existing.

Yes, that looks better. Can do if we agree on this approach.

> 
> > +    }
> > +
> > +    if (!hwclock_available) {
> > +        error_setg(errp, QERR_UNSUPPORTED);
> > +        return;
> > +    }
> >  
> >      /* If user has passed a time, validate and set it. */
> >      if (has_time) {
> > @@ -195,7 +213,7 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
> >  
> >          /* Use '/sbin/hwclock -w' to set RTC from the system time,
> >           * or '/sbin/hwclock -s' to set the system time from RTC. */
> > -        execle("/sbin/hwclock", "hwclock", has_time ? "-w" : "-s",
> > +        execle(hwclock_path, "hwclock", has_time ? "-w" : "-s",
> >                 NULL, environ);
> >          _exit(EXIT_FAILURE);
> >      } else if (pid < 0) {
> > -- 
> > 2.21.0
> > 
> >   
> 
> Regards,
> Daniel