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Date:   Fri, 28 Jul 2023 14:58:51 +0200
From:   Andrew Jones <ajones@ventanamicro.com>
To:     Jisheng Zhang <jszhang@kernel.org>
Cc:     Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] riscv: vdso.lds.S: remove hardcoded 0x800 .text
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Message-ID: <20230728-628baf3727ffccddae0b0663@orel>
References: <20230726173024.3684-1-jszhang@kernel.org>
 <20230726173024.3684-4-jszhang@kernel.org>
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On Thu, Jul 27, 2023 at 01:30:24AM +0800, Jisheng Zhang wrote:
> I believe the hardcoded 0x800 and related comments come from the long
> history VDSO_TEXT_OFFSET in x86 vdso code, but commit 5b9304933730
> ("x86 vDSO: generate vdso-syms.lds") and commit f6b46ebf904f ("x86
> vDSO: new layout") removes the comment and hard coding for x86.
> 
> Similar as x86 and other arch, riscv doesn't need the rigid layout
> using VDSO_TEXT_OFFSET since it "no longer matters to the kernel".
> so we could remove the hard coding now, and removing it brings a
> small vdso.so and aligns with other architectures.
> 
> Also, having enough separation between data and text is important for
> I-cache, so similar as x86, move .note, .eh_frame_hdr, and .eh_frame
> between .rodata and .text.
> 
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> ---
>  arch/riscv/kernel/vdso/vdso.lds.S | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/riscv/kernel/vdso/vdso.lds.S b/arch/riscv/kernel/vdso/vdso.lds.S
> index 671aa21769bc..cbe2a179331d 100644
> --- a/arch/riscv/kernel/vdso/vdso.lds.S
> +++ b/arch/riscv/kernel/vdso/vdso.lds.S
> @@ -23,12 +23,8 @@ SECTIONS
>  	.gnu.version_d	: { *(.gnu.version_d) }
>  	.gnu.version_r	: { *(.gnu.version_r) }
>  
> -	.note		: { *(.note.*) }		:text	:note
>  	.dynamic	: { *(.dynamic) }		:text	:dynamic
>  
> -	.eh_frame_hdr	: { *(.eh_frame_hdr) }		:text	:eh_frame_hdr
> -	.eh_frame	: { KEEP (*(.eh_frame)) }	:text
> -
>  	.rodata		: {
>  		*(.rodata .rodata.* .gnu.linkonce.r.*)
>  		*(.got.plt) *(.got)
> @@ -37,13 +33,16 @@ SECTIONS
>  		*(.bss .bss.* .gnu.linkonce.b.*)
>  	}
>  
> +	.note		: { *(.note.*) }		:text	:note
> +
> +	.eh_frame_hdr	: { *(.eh_frame_hdr) }		:text	:eh_frame_hdr
> +	.eh_frame	: { KEEP (*(.eh_frame)) }	:text
> +
>  	/*
> -	 * This linker script is used both with -r and with -shared.
> -	 * For the layouts to match, we need to skip more than enough
> -	 * space for the dynamic symbol table, etc. If this amount is
> -	 * insufficient, ld -shared will error; simply increase it here.
> +	 * Text is well-separated from actual data: there's plenty of
> +	 * stuff that isn't used at runtime in between.
>  	 */
> -	. = 0x800;
> +	. = ALIGN(16);
>  	.text		: { *(.text .text.*) }		:text
>  
>  	. = ALIGN(4);
> -- 
> 2.40.1
>

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>

Thanks,
drew