linux-kernel - Re: [PATCH] RISC-V: Make BSS section as the last section in vmlinux.lds.S

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Date:   Tue, 18 Dec 2018 13:53:23 +0530
From:   Anup Patel <anup@...infault.org>
To:     Nick Kossifidis <mick@....forth.gr>
Cc:     Palmer Dabbelt <palmer@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christoph Hellwig <hch@...radead.org>,
        Atish Patra <atish.patra@....com>,
        linux-riscv@...ts.infradead.org,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] RISC-V: Make BSS section as the last section in vmlinux.lds.S

On Mon, Dec 17, 2018 at 6:29 PM Nick Kossifidis <mick@....forth.gr> wrote:
>
> Στις 2018-12-17 11:36, Anup Patel έγραψε:
> > On Mon, Nov 26, 2018 at 11:42 AM Anup Patel <anup@...infault.org>
> > wrote:
> >>
> >> The objcopy only emits loadable sections when creating flat kernel
> >> Image. To have minimal possible size of flat kernel Image, we should
> >> have all non-loadable sections after loadable sections.
> >>
> >> Currently, execption table section (loadable section) is after BSS
> >> section (non-loadable section) in the RISC-V vmlinux.lds.S. This
> >> is not optimal for having minimal flat kernel Image size hence this
> >> patch makes BSS section as the last section in RISC-V vmlinux.lds.S.
> >>
> >> In addition, we make BSS section aligned to 16byte instead of PAGE
> >> aligned which further reduces flat kernel Image size by few KBs.
> >>
> >> The flat kernel Image size of Linux-4.20-rc4 using GCC 8.2.0 is
> >> 8819980 bytes with current RISC-V vmlinux.lds.S and it reduces to
> >> 7991740 bytes with this patch applied using GCC 8.2.0. In summary,
> >> this patch reduces Linux-4.20-rc4 flat kernel Image size by 809 KB.
> >>
> >> Signed-off-by: Anup Patel <anup@...infault.org>
> >> ---
> >>  arch/riscv/kernel/vmlinux.lds.S | 4 ++--
> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/arch/riscv/kernel/vmlinux.lds.S
> >> b/arch/riscv/kernel/vmlinux.lds.S
> >> index 65df1dfdc303..cc99eed44931 100644
> >> --- a/arch/riscv/kernel/vmlinux.lds.S
> >> +++ b/arch/riscv/kernel/vmlinux.lds.S
> >> @@ -74,8 +74,6 @@ SECTIONS
> >>                 *(.sbss*)
> >>         }
> >>
> >> -       BSS_SECTION(PAGE_SIZE, PAGE_SIZE, 0)
> >> -
> >>         EXCEPTION_TABLE(0x10)
> >>         NOTES
> >>
> >> @@ -83,6 +81,8 @@ SECTIONS
> >>                 *(.rel.dyn*)
> >>         }
> >>
> >> +       BSS_SECTION(0x10, 0x10, 0x10)
> >> +
> >>         _end = .;
> >>
> >>         STABS_DEBUG
> >> --
> >> 2.17.1
> >>
> >
> > Hi All,
> >
> > Any comment on this patch?
> >
> > Regards,
> > Anup
> >
>
> Just a note on coding style, you should be using a macro instead of 0x10
> so that those who read the code can understand what it is and also a few
> comments since searching through the commit logs to understand why you
> used it isn't optimal.

RISC-V can support 32bit, 64bit and 128bit machine-word sizes.

The 0x10 number is the machine-word size in bytes for 128bit
RISC-V CPU (i.e. maximum machine-word size).

I will add a macro MAX_BYTES_PER_LONG in vmlinux.lds.S and
use it in-place of 0x10

Regards,
Anup