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

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

[Anup Patel]

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@brainfault.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

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

[Anup Patel]

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@brainfault.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


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

[Bin Meng]

On Mon, Nov 26, 2018 at 2:12 PM Anup Patel <anup@brainfault.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@brainfault.org>
> ---
>  arch/riscv/kernel/vmlinux.lds.S | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>

Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>

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

[Bin Meng]

On Mon, Nov 26, 2018 at 2:12 PM Anup Patel <anup@brainfault.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@brainfault.org>
> ---
>  arch/riscv/kernel/vmlinux.lds.S | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>

Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>

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

[Anup Patel]

On Mon, Nov 26, 2018 at 11:42 AM Anup Patel <anup@brainfault.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@brainfault.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

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

[Nick Kossifidis]

Στις 2018-12-17 11:36, Anup Patel έγραψε:
> On Mon, Nov 26, 2018 at 11:42 AM Anup Patel <anup@brainfault.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@brainfault.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.

Regards,
Nick


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

[Anup Patel]

On Mon, Dec 17, 2018 at 6:29 PM Nick Kossifidis <mick@ics.forth.gr> wrote:
>
> Στις 2018-12-17 11:36, Anup Patel έγραψε:
> > On Mon, Nov 26, 2018 at 11:42 AM Anup Patel <anup@brainfault.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@brainfault.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

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

[Palmer Dabbelt]

On Mon, 17 Dec 2018 01:36:45 PST (-0800), anup@brainfault.org wrote:
> On Mon, Nov 26, 2018 at 11:42 AM Anup Patel <anup@brainfault.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@brainfault.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?

Sorry, I haven't had a chance to look at it yet.  Aside from the macro issue 
mentioned by Nick it seems OK.

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

[Anup Patel]

On Fri, Dec 21, 2018 at 2:10 AM Palmer Dabbelt <palmer@sifive.com> wrote:
>
> On Mon, 17 Dec 2018 01:36:45 PST (-0800), anup@brainfault.org wrote:
> > On Mon, Nov 26, 2018 at 11:42 AM Anup Patel <anup@brainfault.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@brainfault.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?
>
> Sorry, I haven't had a chance to look at it yet.  Aside from the macro issue
> mentioned by Nick it seems OK.

I have send v2 to address Nick's comment.

Regards,
Anup

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