linux-kernel - Re: [PATCH 10/18] arm64: Introduce FIQ support

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Message-ID: <2bfebb31-afb5-88a7-d092-87f88aa7367a@marcan.st>
Date:   Sun, 7 Feb 2021 17:47:23 +0900
From:   Hector Martin 'marcan' <marcan@...can.st>
To:     Marc Zyngier <maz@...nel.org>
Cc:     soc@...nel.org, linux-arm-kernel@...ts.infradead.org,
        robh+dt@...nel.org, Arnd Bergmann <arnd@...nel.org>,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        Olof Johansson <olof@...om.net>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>
Subject: Re: [PATCH 10/18] arm64: Introduce FIQ support

On 07/02/2021 00.37, Marc Zyngier wrote:
> See my digression in patch 8. I really wonder what the benefit is to
> treat FIQ independently of IRQ, and we might as well generalise
> this. We could always panic on getting a FIQ on platforms that don't
> expect one.
> 
> It'd be good to rope in the other interested parties (Mark for the
> early entry code, James for RAS and SError handling).

CCing Mark and James: TL;DR what do you think about unconditionally 
keeping DAIF.I == DAIF.F, would this break other platforms with spurious 
FIQs or conversely mask FIQs when we don't want to in some cases? The 
FIQ vector would remain a panic except on platforms that require using 
it, via an alternatives patch.

>>   	kernel_ventry	1, sync				// Synchronous EL1h
>>   	kernel_ventry	1, irq				// IRQ EL1h
>> -	kernel_ventry	1, fiq_invalid			// FIQ EL1h
>> +							// FIQ EL1h
>> +	kernel_ventry	1, fiq_invalid, 64, irq, ARM64_NEEDS_FIQ
> 
> It could be better to create a set of first class FIQ handlers rather
> than this alternative target macro. I quickly hacked this instead,
> which I find more readable.

I think I ended up with the macro change to keep it 1:1 with IRQ, vs a 
separate branch... but I didn't think of the fallthrough-with-nop trick, 
neat. It is definitely is more readable. Are you OK with me pulling this 
patch in for v2, with your name on it?

> -	kernel_ventry	0, fiq_invalid_compat, 32	// FIQ 32-bit EL0
> +	kernel_ventry	0, fiq, 32			// FIQ 32-bit EL0

fiq_compat here, right?

-- 
Hector Martin "marcan" (marcan@...can.st)
Public Key: https://mrcn.st/pub