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

[Nicola Vetrini <nicola.vetrini@bugseng.com>]

On 2026-05-06 11:48, Andrew Cooper wrote:
> 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@suse.com>
>>> ---
>>> 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*
> 

Patches welcome :)

-- 
Nicola Vetrini, B.Sc.
Software Engineer
BUGSENG (https://bugseng.com)
LinkedIn: https://www.linkedin.com/in/nicola-vetrini-a42471253