Re: [PATCH] hvm: Correct RTC time offset update error due to tm->tm_year

[annie li <annie.li@oracle.com>]

Thanks a lot for your reply, Ian.
I guess there is misunderstanding here, see following,
> The Linux kernel's version of mktime and Xen's version both differ from
> standard C/POSIX defined function (in fact I expect Xen's is derived
> from Linux's). Not least because they take a bunch of values instead of
> a struct tm as arguments (i.e. they take unsigned int year, not
> tm->tm_year).
>   
Yes, Xen's is same as Linux's.
> If you wanted to compare Xen vs. a POSIX compliant mktime you'd probably
> want the libc version. The Xen and Linux mktime()s certainly differ
> substantially from the eglibc one.
>   
Thanks, my original aim is to address an issue in rtc_set_time of 
\xen\arch\x86\hvm\rtc.c. (at least I thought it was, need your 
confirmation :-) )
> I don't think you can expect that the in-kernel mktime necessarily has
> the same interface as POSIX documents, there is certainly no particular
> reason why they should or must (a kernel is not a POSIX environment).
> The comment preceding Xen's mktime makes no mention of offsetting
> anything by 1900 (or anything else) and I believe its implementation is
> consistent with its defined interface.
>   
Yes, the comments does not mention of offsetting anything by 1900, and 
this is the reason why I created the patch.
In \xen\arch\x86\hvm\rtc.c, rtc_set_time call mktime to calculate the 
seconds since 1970/01/01, the input parameters of mktime are required to 
be in normal date format. Such as: year=1980, mon=12, day=31, hour=23, 
min=59, sec=59. (Just like Linux)

However, current xen code has some problem when dealing with 
tm->tm_year, see following,
    tm->tm_year = from_bcd(s, s->hw.cmos_data[RTC_YEAR]) + 100;
    after = mktime(tm->tm_year, tm->tm_mon + 1, tm->tm_mday,
                   tm->tm_hour, tm->tm_min, tm->tm_sec);
(For example, if current time is 2012/12/31, tm->tm_year is 112 here)

To meet the requirement of Xen's mktime, tm->tm_year should be changed 
to tm->tm_year+1900 when calling mktime in Xen. See following,
    after = mktime(tm->tm_year+1900, tm->tm_mon+1, tm->tm_mday,
                   tm->tm_hour, tm->tm_min, tm->tm_sec);

Thanks,
Annie
> Ian.
>
>   
>> See following diff file which is created between mktime of linux and 
>> mktime of xen, (I did some tab/space format changes for comparison)
>>
>> diff linux-mktime.c xen-mktime.c
>> 2,4c2,4
>> < mktime(const unsigned int year0, const unsigned int mon0,
>> <       const unsigned int day, const unsigned int hour,
>> <       const unsigned int min, const unsigned int sec)
>> ---
>>  > mktime (unsigned int year, unsigned int mon,
>>  >       unsigned int day, unsigned int hour,
>>  >       unsigned int min, unsigned int sec)
>> 6,8c6
>> <       unsigned int mon = mon0, year = year0;
>> <
>> <       /* 1..12 -> 11,12,1..10 */
>> ---
>>  >       /* 1..12 -> 11,12,1..10: put Feb last since it has a leap day. */
>> 10c8
>> <               mon += 12;      /* Puts Feb last since it has leap day */
>> ---
>>  >               mon += 12;
>> 21d18
>> <
>>
>> Thanks
>> Annie
>>     
>>> best regards
>>> yang
>>>
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>
>