linux-kernel - Re: [PATCH] arm64: assembler: Fine-tune code to improve code readability

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Date:   Mon, 21 Mar 2022 13:18:05 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Zhiyuan Dai <daizhiyuan@...tium.com.cn>, catalin.marinas@....com,
        will@...nel.org
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] arm64: assembler: Fine-tune code to improve code
 readability

On 2022-03-21 02:54, Zhiyuan Dai wrote:
> This patch tweaks the code to improve readability.

...but it also changes what the code actually does :(

> Defined values are:
>   PMUVer == 0b0000 : Performance Monitors Extension not implemented.

Plus all the other values, but crucially up to:

"0b1111 IMPLEMENTATION DEFINED form of performance monitors supported, 
PMUv3 not supported."

> Signed-off-by: Zhiyuan Dai <daizhiyuan@...tium.com.cn>
> ---
>   arch/arm64/include/asm/assembler.h | 9 ++++-----
>   1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
> index 6ebdc0f..efd43a1 100644
> --- a/arch/arm64/include/asm/assembler.h
> +++ b/arch/arm64/include/asm/assembler.h
> @@ -489,11 +489,10 @@
>    */
>   	.macro	reset_pmuserenr_el0, tmpreg
>   	mrs	\tmpreg, id_aa64dfr0_el1
> -	sbfx	\tmpreg, \tmpreg, #ID_AA64DFR0_PMUVER_SHIFT, #4

This extracts a *signed* 4-bit field...

> -	cmp	\tmpreg, #1			// Skip if no PMU present

...so this rejects values of both 0 and -1.

> -	b.lt	9000f
> -	msr	pmuserenr_el0, xzr		// Disable PMU access from EL0
> -9000:
> +	ubfx	\tmpreg, \tmpreg, #ID_AA64DFR0_PMUVER_SHIFT, #4
> +	cbz	\tmpreg, .Lskip_pmu_\@		// Skip if no PMU present
> +	msr_s   SYS_PMUSERENR_EL0, xzr	// Disable PMU access from EL0

How confident are you that it's safe to touch a PMUv3 register in an 
unknown implementation which explicitly *isn't* PMUv3?

Thanks,
Robin.

> +.Lskip_pmu_\@:
>   	.endm
>   
>   /*