Re: [PATCH] x86/time: make do_settime() uses more accurate

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 06/05/2026 10:47 am, Andrew Cooper wrote:
> On 06/05/2026 10:35 am, Jan Beulich wrote:
>> As a comment next to one of the invocations states, get_wallclock_time()
>> can take over a second. The order of evaluation of function arguments is
>> in principle unspecified; in practice at least gcc looks to be evaluating
>> them from last to first. Hence with NOW() invoked first, the respective
>> value passed to do_settime() can be off by over a second (which is in
>> contrast to __get_cmos_time() attempting to get the time exactly after an
>> update, i.e. [pretty] precisely at a seconds boundary).
>>
>> This also addresses a Misra C:2012 rule 13.2 ("The value of an expression
>> and its persistent side-effects shall be the same under all permitted
>> evaluation orders") violation each.
>>
>> Fixes: f64134cdb81c ("x86: Fix time_resume() to notify all domains of 
>> wallclock change")
>> Fixes: 0bfcf984b727 ("x86: Reintroduce clocksource=tsc")
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> Of course the time it takes to do all the CMOS reads (or whichever else
>> wallclock time source is in use) also results in an inaccuracy. For
>> __get_cmos_time() this might be solvable by having it latch NOW() before
>> doing the 6 reads, but in particular for efi_get_time() there's hardly
>> anything we can do.
>>
>> As to Misra rule 13.2: tagging.ecl lists the rule as clean. I also can't
>> find any deviation for the two instances fixed here. What am I missing?
> From deviations.ecl:
>
> -doc_begin="The following file is imported from Linux: ignore for now."
> -file_tag+={adopted_time_r8_3,"^xen/arch/x86/time\\.c$"}
>
> I've said before and it bears repeating.  This claim is false and should
> never have been put in to start with, and time.c is not impacted file.

is not the only*

~Andrew