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

[Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>]

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@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*
Patches welcome :)

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