Re: [RFC PATCH] riscv: vdso: remove hardcoded 0x800 .text section start addr

From: Jisheng Zhang <jszhang@kernel.org>
To: Andrew Jones <ajones@ventanamicro.com>
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: [RFC PATCH] riscv: vdso: remove hardcoded 0x800 .text section start addr
Date: Fri, 25 Nov 2022 00:11:05 +0800	[thread overview]
Message-ID: <Y3+XmWazrB0RURfI@xhacker> (raw)
In-Reply-To: <20221124103255.33prhf3gf6xtwlsk@kamzik>

On Thu, Nov 24, 2022 at 11:32:55AM +0100, Andrew Jones wrote:
> On Thu, Nov 24, 2022 at 12:18:05AM +0800, Jisheng Zhang wrote:
> > It seems the hardcoded 0x800 isn't necessary, but removing it brings a
> 
> s/, but/and/
> 
> > small vdso.so and aligns with other architectures.
> 
> This commit message didn't really satisfy my desire to understand why
> the comment and '. = 0x800' were there in the first place and if its safe
> to remove now, so I tried to do some of my own digging. I found
> 
> commit 5b9304933730 ("x86 vDSO: generate vdso-syms.lds")
> commit f6b46ebf904f ("x86 vDSO: new layout")
> 
> which removes the comment and hard coding for x86 by changing the vdso
> Makefile. Then looking at
> 
> commit 9031fefde6f2 ("arm64: VDSO support")
> 
> we see that it starts with the new Makefile approach and doesn't bother
> with the hard coding from the start. As riscv also started with the new
> Makefile approach it also could have dropped the hard coding from the
> start (I guess).
> 
> > 
> > Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> > ---
> >  arch/riscv/kernel/vdso/vdso.lds.S | 8 +-------
> >  1 file changed, 1 insertion(+), 7 deletions(-)
> > 
> > diff --git a/arch/riscv/kernel/vdso/vdso.lds.S b/arch/riscv/kernel/vdso/vdso.lds.S
> > index 150b1a572e61..7be7e618d59c 100644
> > --- a/arch/riscv/kernel/vdso/vdso.lds.S
> > +++ b/arch/riscv/kernel/vdso/vdso.lds.S
> > @@ -31,13 +31,7 @@ SECTIONS
> >  
> >  	.rodata		: { *(.rodata .rodata.* .gnu.linkonce.r.*) }
> >  
> > -	/*
> > -	 * 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.
> > -	 */
> > -	. = 0x800;
> > +	. = ALIGN(4);
> 
> I realize 4 is used here now because I questioned the 16, but after doing
> my digging I think a larger alignment may be better. Loading the text may
> be done with 8 byte or larger reads, so having the section aligned to a
> larger size would be better reading it. We might as well use 16, like
> arm64 does, and like you had before?
> 
> Also, having enough separation between data and text seems to be
> important for cache reasons, based on the comment in
> ./arch/x86/entry/vdso/vdso-layout.lds.S and other vdso history.
> Maybe we should move .note, .eh_frame_hdr, and .eh_frame below
> .rodata like x86 has it?
> 

Thank you so much for pointing out the two commits and above
separation, new version will be sent out soon.

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