Re: [RESEND PATCH] clocksource/arm_arch_timer: Fix masking for high freq counters

[Oliver Upton]

Hi Marc,

On Sat, Aug 7, 2021 at 3:52 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> Hi Oliver,
>
> On Fri, 06 Aug 2021 19:21:26 +0100,
> Oliver Upton <oupton@xxxxxxxxxx> wrote:
> >
> > Unfortunately, the architecture provides no means to determine the bit
> > width of the system counter. However, we do know the following from the
> > specification:
> >
> >  - the system counter is at least 56 bits wide
> >  - Roll-over time of not less than 40 years
> >
> > To date, the arch timer driver has depended on the first property,
> > assuming any system counter to be 56 bits wide and masking off the rest.
> > However, combining a narrow clocksource mask with a high frequency
> > counter could result in prematurely wrapping the system counter by a
> > significant margin. For example, a 56 bit wide, 1GHz system counter
> > would wrap in a mere 2.28 years!
> >
> > This is a problem for two reasons: v8.6+ implementations are required to
> > provide a 64 bit, 1GHz system counter. Furthermore, before v8.6,
> > implementers may select a counter frequency of their choosing.
> >
> > Fix the issue by deriving a valid clock mask based on the second
> > property from above. Set the floor at 56 bits, since we know no system
> > counter is narrower than that.
> >
> > Suggested-by: Marc Zyngier <maz@xxxxxxxxxx>
> > Signed-off-by: Oliver Upton <oupton@xxxxxxxxxx>
> > ---
> > This patch was tested with QEMU, tweaked to provide a 1GHz system
> > counter frequency, as I could not easily figure out how to tweak the
> > base FVP to provide a 1GHz counter.
>
> <FVP>
> "bp.refcounter.base_frequency" is the property you are looking for. In
> general, passing --list-params to the model reveals a treasure trove
> of weird and wonderful options that can be used to configure the model
> to your liking.
> </FVP>

Ah! Thanks for the tip. I didn't find the fast models documentation to
be too terribly helpful.
> >
> > +/*
> > + * The minimum amount of time a generic timer is guaranteed to not roll over
>
> nit: s/timer/counter/
>

Ack.

> > + * (40 years)
>
> For later reference, could you add the section of the ARMv8 ARM where
> this is mentioned? Something like 'ARM DDI 0487G.a D11.1.2 ("The
> system counter")', either here or in the comment further down.
>

Sure thing. I think I'll drop it on the function below since we depend
on two properties of the system counter there.

> > +static int __init arch_counter_get_width(void)
> > +{
> > +     u64 min_cycles = MIN_ROLLOVER_SECS * arch_timer_get_cntfrq();
>
> That's unfortunately wishful thinking. We have stupidly broken systems
> out there that do not set CNTFRQ_EL0, and instead rely on DT
> properties to describe the counter frequency. You're likely to end up
> with a glorious zero as a result, with interesting consequences...

Blech. I didn't read the DT hooks for the driver.  That's awful.

> Use arch_timer_rate instead, which will be set as by the time you
> register the counter.

Ack.

>
> > +
> > +     /* guarantee the returned width is within the valid range */
> > +     return max(56, min(64, ilog2(min_cycles)));
>
> Maybe better written as "clamp_val(ilog2(min_cycles), 56, 64);".
>

Agreed.

> > +     width = arch_counter_get_width();
> > +     clocksource_counter.mask = CLOCKSOURCE_MASK(width);
> > +     cyclecounter.mask = CLOCKSOURCE_MASK(width);
>
> Since you move this to be computed at runtime, how about dropping the
> static initialisation of the mask fields?
>

Yep, already made the change locally after mailing out :)

> > +
> >       if (!arch_counter_suspend_stop)
> >               clocksource_counter.flags |= CLOCK_SOURCE_SUSPEND_NONSTOP;
> >       start_count = arch_timer_read_counter();
> > @@ -1040,8 +1065,7 @@ static void __init arch_counter_register(unsigned type)
> >       timecounter_init(&arch_timer_kvm_info.timecounter,
> >                        &cyclecounter, start_count);
> >
> > -     /* 56 bits minimum, so we assume worst case rollover */
> > -     sched_clock_register(arch_timer_read_counter, 56, arch_timer_rate);
> > +     sched_clock_register(arch_timer_read_counter, width, arch_timer_rate);
> >  }
> >
> >  static void arch_timer_stop(struct clock_event_device *clk)
>
> Another thing that needs addressing for high frequency counter support
> is to move away from TVAL programming and switch to CVAL, as the
> maximum deadline we can currently program is 4.2s at 1GHz.
>
> Fun fact: it has the interesting consequence of breaking XGene-1,
> which implemented CVAL in terms of TVAL instead of the other way
> around (what were these guys thinking?)

Yikes!

> , though I don't think anyone
> will notice in practice. I have a preliminary patch on a branch
> somewhere that I'll try to dust up and post in the coming days.

Sounds good, I'll clean this one up and send it out too.

--
Thanks,
Oliver