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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Nov 28, 2019 at 01:49:32PM +0100, Cornelia Huck wrote:
> 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.

I'm fine with this


Regards,
Daniel
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