linux-kernel - Re: [PATCH 1/6] arm64: scs: Store absolute SCS stack pointer value in thread

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Date:   Mon, 18 May 2020 12:37:10 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@...com>,
        Jann Horn <jannh@...gle.com>, Ard Biesheuvel <ardb@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, kernel-team@...roid.com
Subject: Re: [PATCH 1/6] arm64: scs: Store absolute SCS stack pointer value
 in thread_info

On Fri, May 15, 2020 at 06:27:51PM +0100, Will Deacon wrote:
> Storing the SCS information in thread_info as a {base,offset} pair
> introduces an additional load instruction on the ret-to-user path,
> since the SCS stack pointer in x18 has to be converted back to an offset
> by subtracting the base.
> 
> Replace the offset with the absolute SCS stack pointer value instead
> and avoid the redundant load.
> 
> Signed-off-by: Will Deacon <will@...nel.org>

One trivial nit below, but regardless this looks sound to me, and I
certainly prefer having the absolute address rather than an offset, so:

Reviewed-by: Mark Rutland <mark.rutland@....com>

> diff --git a/kernel/scs.c b/kernel/scs.c
> index 9389c28f0853..5ff8663e4a67 100644
> --- a/kernel/scs.c
> +++ b/kernel/scs.c
> @@ -60,8 +60,7 @@ int scs_prepare(struct task_struct *tsk, int node)
>  	if (!s)
>  		return -ENOMEM;
>  
> -	task_scs(tsk) = s;
> -	task_scs_offset(tsk) = 0;
> +	task_scs(tsk) = task_scs_sp(tsk) = s;

I think this would be more legible as two statements:

|	task_sys(tsk) = s;
|	task_scs_sp(tsk) = s;

... as we usually save `foo = bar = baz` stuff for the start of a
function or within loop conditions.

Thanks,
Mark.

>  	scs_account(tsk, 1);
>  	return 0;
>  }
> -- 
> 2.26.2.761.g0e0b3e54be-goog
>