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

From: Jisheng Zhang <jszhang@kernel.org>
To: Palmer Dabbelt <palmer@dabbelt.com>
Cc: ajones@ventanamicro.com, Paul Walmsley <paul.walmsley@sifive.com>,
	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: Mon, 12 Dec 2022 23:21:45 +0800	[thread overview]
Message-ID: <Y5dHCVyCfg7qc3Mb@xhacker> (raw)
In-Reply-To: <mhng-599fbea8-24d0-4d59-896f-7ce09eb6c1c6@palmer-ri-x1c9>

On Fri, Dec 09, 2022 at 11:02:18AM -0800, Palmer Dabbelt wrote:
> On Thu, 24 Nov 2022 08:11:05 PST (-0800), jszhang@kernel.org wrote:
> > 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.
> 
> Not sure if I missed the v2?  I can't find one.

When I want to sent a v2, I just knew that patches after rc7 won't
be merged for next rc1. Sorry for not being earlier. Will send out
v2 once new developement window is reopen.

Thanks