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

[Daniel Thompson <daniel.thompson@linaro.org>]

On 16/07/14 18:21, Harro Haan wrote:
> On 16 July 2014 14:54, Daniel Thompson <daniel.thompson@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.

This is great work.

Be aware that I'd also expect the the performance of my workaround would
drop a little bit when when support for SGIs is added (mostly due to due
to increased code size).