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

[Palmer Dabbelt]

On Thu, 24 Nov 2022 08:11:05 PST (-0800), jszhang at 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 at 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.