linux-kernel - Re: [PATCH 09/15] irq: arm: perform irqentry in entry code

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Message-ID: <78cc83d10cb6d251e53f5947ddc52735@kernel.org>
Date:   Wed, 01 Dec 2021 07:56:29 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Vladimir Murzin <vladimir.murzin@....com>
Cc:     Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org,
        aou@...s.berkeley.edu, catalin.marinas@....com,
        deanbo422@...il.com, green.hu@...il.com, guoren@...nel.org,
        jonas@...thpole.se, kernelfans@...il.com,
        linux-arm-kernel@...ts.infradead.org, linux@...linux.org.uk,
        nickhu@...estech.com, palmer@...belt.com, paulmck@...nel.org,
        paul.walmsley@...ive.com, peterz@...radead.org, shorne@...il.com,
        stefan.kristiansson@...nalahti.fi, tglx@...utronix.de,
        torvalds@...ux-foundation.org, tsbogend@...ha.franken.de,
        vgupta@...nel.org, will@...nel.org
Subject: Re: [PATCH 09/15] irq: arm: perform irqentry in entry code

On 2021-11-30 08:49, Vladimir Murzin wrote:

> One month and a week later...
> 
> I observe that in terms of performance
> 
>    MRS       r0, ipsr
> 
> is equivalent to readl_relaxed(BASEADDR_V7M_SCB + V7M_SCB_ICSR)
> 
>    MOV.W   r3, #3758153728
>    LDR.W   r0, [r3, #3332]
> 
> Old compilers can produce less performant sequence like
> 
>    LDR      r3,0xbcc0
>    ADD.W    r3,r3,#0xaf00
>    LDR      r0,[r3,#0]
> 
> So, what would be your preference?

If there is no significant overhead to reading the MMIO register
and that you see a benefit in enabling COMPILE_TEST, then this
probably is the way to go.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...