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Date:   Tue, 25 Oct 2022 13:31:28 +0100
Message-ID: <86o7u0dqzj.wl-maz@kernel.org>
From:   Marc Zyngier <maz@kernel.org>
To:     Joe Korty <joe.korty@concurrent-rt.com>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] arm64: arch_timer: XGene-1: TVAL register math error breaks timer expiry calculation.
In-Reply-To: <20221024165422.GA51107@zipoli.concurrent-rt.com>
References: <20221024165422.GA51107@zipoli.concurrent-rt.com>
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Hi Joe,

Thanks for respinning this. Some comments below. Nothing major, but
worth keeping in mind for your next patches.

On Mon, 24 Oct 2022 17:54:22 +0100,
Joe Korty <joe.korty@concurrent-rt.com> wrote:
> 
> arm64: arch_timer: XGene-1: TVAL register math error breaks timer expiry calculation.
>

nit: this line is already in the email subject which will be captured
when the patch is applied. No need to repeat it. Also, no final '.' at
the end of the patch subject.

> The TVAL register is 32 bit signed.  Thus only the lower 31 bits are
> available to specify when an interrupt is to occur at some time in the
> near future.  Attempting to specify a larger interval with TVAL results
> in a negative time delta which means the timer fires immediately upon
> being programmed, rather than firing at that expected future time.
> 
> The solution is for linux to declare that TVAL is a 31 bit register rather

nit: s/linux/Linux/

> than give its true size of 32 bits.  This prevents linux from programming
> TVAL with a too-large value.  Note that, prior to 5.16, this little trick
> was the standard way to handle TVAL in linux, so there is nothing new
> happening here on that front.
> 
> Test procedure: for some reason, the lockup watchdog is sensitive to
> this bug.

My interpretation is that the softlockup detector hides the issue,
because it keeps generating short timer deadlines that are within the
scope of the broken timer.

Disable it, and you start using NO_HZ with much longer timer
deadlines, which turns into an interrupt flood:

 11: 1124855130  949168462  758009394   76417474  104782230   30210281     310890 1734323687     GICv2  29 Level     arch_timer

And "much longer" isn't that long: it takes less than 43s to underflow
TVAL at 50MHz (the frequency of the counter on XGene-1).

> When we turn the watchdog off, then run a little 'hello world'
> program in each of the CPUs, there will often be one that 1) hangs up
> forever, 2) hangs up what seems like forever, but actully contines after
> a few minutes.  In either case, the program cannot be freed by a ^C.
> This test sequence requires CONFIG_SOFTLOCKUP_DETECTOR, and probably
> requires that one of the NO_HZ Kconfig options be specified.
> 
> The sequence is, for an 8 cpu Mustang XGene-1:
> 
>    echo 0 >/proc/sys/kernel/watchdog
>    for i in {0..7}; do taskset -c $i echo hi there $i; done
>
> Note that though the hangup usually happens, it does not
> always happen.

The first line is enough to kill the machine here. Nice one! :D

> 
> Some comments on the v1 version of this patch by Marc Zyngier:

nit: mentioning previous versions in the commit message isn't very
helpful, as the git tree won't carry that patch. Just saying
"additional comment from John Doe:" is enough.

> 
>   XGene implements CVAL (a 64bit comparator) in terms of TVAL (a countdown
>   register) instead of the other way around. TVAL being a 32bit register,
>   the width of the counter should equally be 32.  However, TVAL is a
>   *signed* value, and keeps counting down in the negative range once the
>   timer fires.
> 
>   It means that any TVAL value with bit 31 set will fire immediately,
>   as it cannot be distinguished from an already expired timer. Reducing
>   the timer range back to a paltry 31 bits papers over the issue.
> 
>   Another problem cannot be fixed though, which is that the timer interrupt
>   *must* be handled within the negative countdown period, or the interrupt
>   will be lost (TVAL will rollover to a positive value, indicative of a
>   new timer deadline).
> 
>  [ v2: Expanded CC list - jak ]
>  [ v2: Revamped changelog - jak ] 
>  [ v2: streamlined inlined comments - jak ]

This information should be stashed below the '---' line so that it
isn't captured in the commit when applying the patch.

> 
> Cc: stable@vger.kernel.org # 5.16+
> Fixes: 012f18850452 ("clocksource/drivers/arm_arch_timer: Work around broken CVAL implementations")
> Signed-off-by: Joe Korty <joe.korty@concurrent-rt.com>
> ---
> base-commit: v6.0
> Index: b/drivers/clocksource/arm_arch_timer.c
> ===================================================================
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -804,6 +804,9 @@ static u64 __arch_timer_check_delta(void
>  		/*
>  		 * XGene-1 implements CVAL in terms of TVAL, meaning
>  		 * that the maximum timer range is 32bit. Shame on them.
> +		 *
> +		 * Note that TVAL is signed, thus has only 31 of its
> +		 * 32 bits to express magnitude.
>  		 */
>  		MIDR_ALL_VERSIONS(MIDR_CPU_MODEL(ARM_CPU_IMP_APM,
>  						 APM_CPU_PART_POTENZA)),
> @@ -811,8 +814,8 @@ static u64 __arch_timer_check_delta(void
>  	};
>  
>  	if (is_midr_in_range_list(read_cpuid_id(), broken_cval_midrs)) {
> -		pr_warn_once("Broken CNTx_CVAL_EL1, limiting width to 32bits");
> -		return CLOCKSOURCE_MASK(32);
> +		pr_warn_once("Broken CNTx_CVAL_EL1, using 32 bit TVAL instead.\n");

s/TVAL/CNTx_TVAL_EL1/ instead.

> +		return CLOCKSOURCE_MASK(31);
>  	}
>  #endif
>  	return CLOCKSOURCE_MASK(arch_counter_get_width());
> 

With the commit message suitably amended:

Reviewed-by: Marc Zyngier <maz@kernel.org>

Daniel, would you mind fixing it up when applying this patch? XGene is
trivially broken without this fix, and it would be good if it could
make it in one of the 6.1-rc.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.