Re: [PATCH v3] qga: fence guest-set-time if hwclock not available

[Cornelia Huck <cohuck@redhat.com>]

On Thu, 5 Dec 2019 14:05:19 +0100
Philippe Mathieu-Daudé <philmd@redhat.com> wrote:

> Hi Cornelia,
> 
> On 12/5/19 12:53 PM, 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.
> > 
> > Reviewed-by: Laszlo Ersek <lersek@redhat.com>
> > Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
> > Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
> > Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> > ---
> > 
> > v2->v3:
> >    - added 'static' keyword to hwclock_path
> > 
> > Not sure what tree this is going through; if there's no better place,
> > I can also take this through the s390 tree.  
> 
> s390 or trivial trees seems appropriate.
> 
> > 
> > ---
> >   qga/commands-posix.c | 13 ++++++++++++-
> >   1 file changed, 12 insertions(+), 1 deletion(-)
> > 
> > diff --git a/qga/commands-posix.c b/qga/commands-posix.c
> > index 1c1a165daed8..0be301a4ea77 100644
> > --- a/qga/commands-posix.c
> > +++ b/qga/commands-posix.c
> > @@ -156,6 +156,17 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
> >       pid_t pid;
> >       Error *local_err = NULL;
> >       struct timeval tv;
> > +    static const char hwclock_path[] = "/sbin/hwclock";
> > +    static int hwclock_available = -1;
> > +
> > +    if (hwclock_available < 0) {
> > +        hwclock_available = (access(hwclock_path, X_OK) == 0);
> > +    }
> > +
> > +    if (!hwclock_available) {
> > +        error_setg(errp, QERR_UNSUPPORTED);  
> 
> In include/qapi/qmp/qerror.h we have:
> 
> /*
>   * These macros will go away, please don't use in new code, and do not
>   * add new ones!
>   */

Sigh, it is really hard to keep track here :( I just copied from other
callers in this file...

> 
> Maybe we can replace it by "this feature is not supported on this 
> architecture"? (or without 'on this architecture').

This is not really architecture specific, you'd get this on any setup
that does not have /sbin/hwclock.

Q: Is libvirt doing anything with such an error message from QEMU? Do
we have the freedom to say e.g "guest-set-time is not supported" or so?
Or is it beneficial to print the same error message for any unsupported
feature?

> 
> > +        return;
> > +    }
> >   
> >       /* If user has passed a time, validate and set it. */
> >       if (has_time) {
> > @@ -195,7 +206,7 @@ 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) {
> >   
>