[PATCH v8 0/4] arm: KGDB NMI/FIQ support

[Harro Haan]

On 16 July 2014 14:54, Daniel Thompson <daniel.thompson at linaro.org> wrote:
> On 15/07/14 19:45, Marek Vasut wrote:
>>>> I can reduce the number of occurrences (not prevent it) by adding the
>>>> following hack to irq-gic.c
>>>> @@ -297,10 +309,12 @@ static asmlinkage void __exception_irq_entry
>>>> gic_handle_irq(struct pt_regs *regs
>>>>
>>>>   u32 irqstat, irqnr;
>>>>   struct gic_chip_data *gic = &gic_data[0];
>>>>   void __iomem *cpu_base = gic_data_cpu_base(gic);
>>>>
>>>>   do {
>>>>
>>>> + while(readl_relaxed(gic_data_dist_base(gic) + GIC_DIST_PENDING_SET)
>>>> & (1 << 30))
>>>> +   printk(KERN_ERR "TEMP: gic_handle_irq: wait for FIQ exception\n");
>>>>
>>>>   irqstat = readl_relaxed(cpu_base + GIC_CPU_INTACK);
>>>>   irqnr = irqstat & ~0x1c00;
>>>
>>> I've made a more complete attempt to fix this. Could you test the
>>> following? (and be prepared to fuzz the line numbers)
>>
>> There's also another workaround, look at [1], but it's really a perverse hack
>> thus far (blush). What I did there is I got hinted that an L1 page table can
>> have this NS bit set. If this bit is set for a mapping, all accesses to memory
>> area via that mapping will be non-secure. And then, in turn, by doing a non-
>> secure read of the INTACK register, it will not ever happen that the FIQ number
>> will pop up in the INTACK. I only do a non-secure read of the INTACK register,
>> all other registers of the GICv1 are read via regular secure-mode accesses.
>
> I'll be looking into this approach.
>
> It is technically a better approach than mine since it prevents the IRQ
> handler from ever reading a group 0 interrupt from INTACK.

Agree, preventing the problem is better than fixing it afterwards.

>
> Unfortunately the tentacles of this workaround reach pretty deep in the
> memory management code (rather than being concentrated in the GIC
> driver) but the improved runtime behaviour might be worth it.

I did some worst case measurements on the SabreSD while running:
$ while true; do hackbench 20; done &

Use banked non-secure GIC_CPU_INTACK register for regular interrupts
(patches by Marek):
The FIQ handler reads the TWD_TIMER_COUNTER 2570 ticks (which is x
1000 / 498 = 5161 nsec) after FIQ interrupt ID30 is generated.
The average is around 497 ticks.
The minimum is around 34 ticks.

Use re-trigger approach by putting it back to pending state (latest
patch by Daniel):
The FIQ handler reads the TWD_TIMER_COUNTER 2678 ticks (which is x
1000 / 498 = 5378 nsec) after FIQ interrupt ID30 is generated.
The average is around 563 ticks (note: almost everything is normal path)
The minimum is around 34 ticks (note: this is the normal path, not the
re-trigger path)

So the results are quite similar.