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SubjectRe: [RFC PATCH 17/23] watchdog/hardlockup/hpet: Convert the timer's interrupt to NMI
On Tue, Jun 12, 2018 at 05:57:37PM -0700, Ricardo Neri wrote:

+static bool is_hpet_wdt_interrupt(struct hpet_hld_data *hdata)
+{
+ unsigned long this_isr;
+ unsigned int lvl_trig;
+
+ this_isr = hpet_readl(HPET_STATUS) & BIT(hdata->num);
+
+ lvl_trig = hpet_readl(HPET_Tn_CFG(hdata->num)) & HPET_TN_LEVEL;
+
+ if (lvl_trig && this_isr)
+ return true;
+
+ return false;
+}

> +static int hardlockup_detector_nmi_handler(unsigned int val,
> + struct pt_regs *regs)
> +{
> + struct hpet_hld_data *hdata = hld_data;
> + unsigned int use_fsb;
> +
> + /*
> + * If FSB delivery mode is used, the timer interrupt is programmed as
> + * edge-triggered and there is no need to check the ISR register.
> + */
> + use_fsb = hdata->flags & HPET_DEV_FSB_CAP;

Please do explain.. That FSB thing basically means MSI. But there's only
a single NMI vector. How do we know this NMI came from the HPET?

> +
> + if (!use_fsb && !is_hpet_wdt_interrupt(hdata))

So you add _2_ HPET reads for every single NMI that gets triggered...
and IIRC HPET reads are _sllooooowwwwww_.

> + return NMI_DONE;
> +
> + inspect_for_hardlockups(regs);
> +
> + if (!(hdata->flags & HPET_DEV_PERI_CAP))
> + kick_timer(hdata);
> +
> + /* Acknowledge interrupt if in level-triggered mode */
> + if (!use_fsb)
> + hpet_writel(BIT(hdata->num), HPET_STATUS);
> +
> + return NMI_HANDLED;

So if I read this right, when in FSB/MSI mode, we'll basically _always_
claim every single NMI as handled?

That's broken.

> +}

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