linux-kernel - Re: [PATCH 3/3] arm64: reduce trampoline data alignment

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Message-ID: <20200317223433.GL20788@willie-the-truck>
Date:   Tue, 17 Mar 2020 22:34:33 +0000
From:   Will Deacon <will@...nel.org>
To:     Rémi Denis-Courmont <remi@...lab.net>
Cc:     catalin.marinas@....com, linux-arm-kernel@...ts.infradead.org,
        mark.rutland@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] arm64: reduce trampoline data alignment

On Mon, Mar 16, 2020 at 02:40:46PM +0200, Rémi Denis-Courmont wrote:
> From: Rémi Denis-Courmont <remi.denis.courmont@...wei.com>
> 
> The trampoline data, currently consisting of two relocated pointers,
> must be within a single page. However, there are no needs for it to
> start a page.
> 
> This reduces the alignment to 16 bytes (with SDEI) or 8 bytes (without
> SDEI), which is sufficient to ensure that the data is entirely within a
> single page of the fixmap.
> 
> Signed-off-by: Rémi Denis-Courmont <remi.denis.courmont@...wei.com>
> ---
>  arch/arm64/kernel/entry.S | 4 ++--
>  arch/arm64/mm/mmu.c       | 5 ++---
>  2 files changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index af17fcb4aaea..b648f9fe1e33 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -858,12 +858,12 @@ SYM_CODE_END(tramp_exit_compat)
>  	.popsection				// .entry.tramp.text
>  #ifdef CONFIG_RANDOMIZE_BASE
>  	.pushsection ".rodata", "a"
> -	.align PAGE_SHIFT
>  #ifdef CONFIG_ARM_SDE_INTERFACE
> +	.align	4	// all .rodata must be in a single fixmap page
>  SYM_DATA_START(__sdei_asm_trampoline_next_handler)
>  	.quad	__sdei_asm_handler
>  SYM_DATA_END(__sdei_asm_trampoline_next_handler)
> -#endif
> +#endif /* CONFIG_ARM_SDE_INTERFACE */
>  SYM_DATA_START(__entry_tramp_data_start)
>  	.quad	vectors
>  SYM_DATA_END(__entry_tramp_data_start)
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 9b08f7c7e6f0..6a0e75f48e7b 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -599,9 +599,8 @@ static int __init map_entry_trampoline(void)
>  	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {
>  		extern char __entry_tramp_data_start[];
>  
> -		__set_fixmap(FIX_ENTRY_TRAMP_DATA,
> -			     __pa_symbol(__entry_tramp_data_start),
> -			     PAGE_KERNEL_RO);
> +		pa_start = __pa_symbol(__entry_tramp_data_start) & PAGE_MASK;
> +		__set_fixmap(FIX_ENTRY_TRAMP_DATA, pa_start, PAGE_KERNEL_RO);
>  	}
>  
>  	return 0;

Acked-by: Will Deacon <will@...nel.org>

Will