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

[Laszlo Ersek]

On 12/05/19 14:05, Philippe Mathieu-Daudé 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 <address@hidden>
>> Reviewed-by: Daniel P. Berrangé <address@hidden>
>> Reviewed-by: Michael Roth <address@hidden>
>> Signed-off-by: Cornelia Huck <address@hidden>
>> ---
>>
>> 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!
>  */

Obviously, the last word on this belongs to Markus (CC'd) -- he added
that comment. I'd just like to point out *when* that comment was added:
approx. four and half years ago. (See commit 4629ed1e9896.)

I've always associated QERR_UNSUPPORTED with QMP interfaces rejecting
invocation due to lack of support. I don't think one more instance of
QERR_UNSUPPORTED will matter much, when we'll "finally" :) convert or
eliminate the other 35! (Yes, I've counted.)

In case it's unacceptable to add one more QERR_UNSUPPORTED: what is the
official solution that replaces it?

I assume it was explained in the series that included commit
4629ed1e9896, but I can't easily tell. (And, there is no "QERR_" match
in docs/.)

Hmmm, more history digging... In the 4629ed1e9896..v4.2.0-rc4 set of
commits, the following commits introduced new instances of
QERR_UNSUPPORTED:

- e09484efbc9d ("qmp: add QMP interface "query-cpu-model-expansion"", 
2016-09-06)
- 0031e0d68339 ("qmp: add QMP interface "query-cpu-model-comparison"", 
2016-09-06)
- b18b6043341d ("qmp: add QMP interface "query-cpu-model-baseline"", 2016-09-06)
- 1007a37e2082 ("smbios: filter based on CONFIG_SMBIOS rather than TARGET", 
2017-01-16)
- 9f57061c3555 ("acpi: filter based on CONFIG_ACPI_X86 rather than TARGET", 
2017-01-16)
- 39164c136cba ("qmp/hmp: add query-vm-generation-id and 'info 
vm-generation-id' commands", 2017-03-02)
- 161a56a9065f ("qga: Add 'guest-get-users' command", 2017-04-26)
- 53c58e64d0a2 ("qga: Add `guest-get-timezone` command", 2017-04-27)
- e674605f9821 ("qemu-ga: check if utmpx.h is available on the system", 
2017-07-17)

I don't claim that all of those additions have stuck with us, to
v4.2.0-rc4. Yet, in general, practice doesn't seem to have followed the
intended deprecation.

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

I think if we replace QERR_UNSUPPORTED with anything, it should be
"similarly standardized". (Lack of support for a given QMP interface is
pretty common, I think.)

Thanks,
Laszlo

>
>> +        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) {
>>
>