linux-kernel - Re: [PATCH] arm64: Fix double TCR_T0SZ

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Date: Tue, 2 Apr 2024 12:58:41 +0100
From: Will Deacon <will@...nel.org>
To: Seongsu Park <sgsu.park@...sung.com>
Cc: catalin.marinas@....com, ardb@...nel.org, mark.rutland@....com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Leem ChaeHoon <infinite.run@...il.com>,
	Gyeonggeon Choi <gychoi@...dent.42seoul.kr>,
	Soomin Cho <to.soomin@...il.com>, DaeRo Lee <skseofh@...il.com>,
	kmasta <kmasta.study@...il.com>
Subject: Re: [PATCH] arm64: Fix double TCR_T0SZ_OFFSET shift

On Tue, Apr 02, 2024 at 07:49:50PM +0900, Seongsu Park wrote:
> We have already shifted the value of t0sz in TCR_T0SZ by TCR_T0SZ_OFFSET.
> So, the TCR_T0SZ_OFFSET shift here should be removed.
> 
> Co-developed-by: Leem ChaeHoon <infinite.run@...il.com>
> Signed-off-by: Leem ChaeHoon <infinite.run@...il.com>
> Co-developed-by: Gyeonggeon Choi <gychoi@...dent.42seoul.kr>
> Signed-off-by: Gyeonggeon Choi <gychoi@...dent.42seoul.kr>
> Co-developed-by: Soomin Cho <to.soomin@...il.com>
> Signed-off-by: Soomin Cho <to.soomin@...il.com>
> Co-developed-by: DaeRo Lee <skseofh@...il.com>
> Signed-off-by: DaeRo Lee <skseofh@...il.com>
> Co-developed-by: kmasta <kmasta.study@...il.com>
> Signed-off-by: kmasta <kmasta.study@...il.com>
> Signed-off-by: Seongsu Park <sgsu.park@...sung.com>

heh, that's quite a lot of people. Did you remove three chars each? :p

> ---
>  arch/arm64/include/asm/mmu_context.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
> index c768d16b81a4..58de99836d2e 100644
> --- a/arch/arm64/include/asm/mmu_context.h
> +++ b/arch/arm64/include/asm/mmu_context.h
> @@ -76,7 +76,7 @@ static inline void __cpu_set_tcr_t0sz(unsigned long t0sz)
>  		return;
>  
>  	tcr &= ~TCR_T0SZ_MASK;
> -	tcr |= t0sz << TCR_T0SZ_OFFSET;
> +	tcr |= t0sz;

Thankfully, TCR_T0SZ_OFFSET is 0 so this isn't as alarming as it looks.
Even so, if we're going to make the code consistent, then shouldn't the
earlier conditional be updated too?

	if ((tcr & TCR_T0SZ_MASK) >> TCR_T0SZ_OFFSET == t0sz)
		return;

seems to assume that t0sz is unshifted.

Will