From mboxrd@z Thu Jan  1 00:00:00 1970
From: will.deacon@arm.com (Will Deacon)
Date: Thu, 9 Aug 2018 14:16:22 +0100
Subject: Bug report: Bad mode in FIQ handler detected on CPU0, code
 0x56000000
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To: linux-arm-kernel@lists.infradead.org
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Hi Chunyu,

On Thu, Aug 09, 2018 at 08:47:38AM -0400, Chunyu Hu wrote:
> I hit an 100% panic when I executed:
> 
> perf &
> perf top
> 
> Logs are as below,
> 
> [  123.234492] Bad mode in FIQ handler detected on CPU0, code 0x56000000 -- SVC (AArch64)

Oh dear...

We don't support FIQ in Linux, since:

(a) It's seldom available on the non-secure side
(b) It's not necessarily "fast", despite the name
(c) We'd be opening up a can of worms if we allow FIQ to preempt IRQ. Better
    off using SDEI or GIC priorities to implement NMIs.
(d) I don't think FIQs can be described in ACPI or DT

In this case, it looks like the PMU interrupt has been configured as a FIQ.
It should be possible to configure the GIC in firmware (not sure you have
EL3, so maybe in the bootloader) to treat this interrupt as an IRQ instead,
in which case perf top will work correctly as long as the ACPI tables / FDT
accurately describe the interrupt.

Will