[RFC PATCH] ld/emulparams: elf32lriscv-defs: Add support tune the text segment start address

[Fangrui Song]

On 2022-12-18, Lad, Prabhakar wrote:
>Hi Fangrui,
>
>Thank you for the feedback.
>
>On Sun, Dec 18, 2022 at 6:24 AM Fangrui Song <i@maskray.me> wrote:
>>
>> On 2022-12-16, Lad Prabhakar via Binutils wrote:
>> >On the RISC-V architecture the TEXT_START_ADDR defaults to 0x10000. On
>> >some RISC-V platforms we want to set this offset to something else. So
>> >this patch provides a way to tune the text segment start address.
>> >elf32lriscv-defs.sh now checks for DEFAULT_TEXT_START_ADDR variable and
>> >if being set it overrides TEXT_START_ADDR to the value set by
>> >DEFAULT_TEXT_START_ADDR or else defaults to 0x10000.
>> >
>> >Renesas RZ/Five RISC-V SoC has Instruction local memory and Data local
>> >memory (ILM & DLM) which maps between region 0x30000 - 0x4FFFF. When the
>> >virtual address falls in this range the MMU doesn't trigger a page
>> >fault and assumes the virtual address as physical address and causes
>> >undesired behaviours of statically applications/libraries. Hence introduce
>> >an option to tune the TEXT_START_ADDR.
>> >
>> >Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
>> >---
>> >Hi All,
>> >
>> >This patch is inspired from the current ld/emulparams/nds32elf_linux.sh file
>> >where similar approach is being used and DEFAULT_TEXT_START_ADDR variable is
>> >checked to adjust the TEXT_START_ADDR for the platform.
>> >
>> >I am not sure if this is the right approach the above issue has been discussed
>> >on the ML [0].
>> >
>> >[0] https://sourceware.org/pipermail/binutils/2022-November/124813.html
>> >
>> >Cheers,
>> >Prabhakar
>> >---
>> > ld/emulparams/elf32lriscv-defs.sh | 6 +++++-
>> > 1 file changed, 5 insertions(+), 1 deletion(-)
>> >
>> >diff --git a/ld/emulparams/elf32lriscv-defs.sh b/ld/emulparams/elf32lriscv-defs.sh
>> >index b823cedacab..026aef4714b 100644
>> >--- a/ld/emulparams/elf32lriscv-defs.sh
>> >+++ b/ld/emulparams/elf32lriscv-defs.sh
>> >@@ -27,7 +27,11 @@ case "$target" in
>> > esac
>> >
>> > IREL_IN_PLT=
>> >-TEXT_START_ADDR=0x10000
>> >+if [ -z ${DEFAULT_TEXT_START_ADDR+x} ]; then
>> >+    TEXT_START_ADDR=0x10000
>> >+else
>> >+    TEXT_START_ADDR=$DEFAULT_TEXT_START_ADDR
>> >+fi
>> > MAXPAGESIZE="CONSTANT (MAXPAGESIZE)"
>> > COMMONPAGESIZE="CONSTANT (COMMONPAGESIZE)"
>> >
>> >--
>> >2.17.1
>> >
>>
>> Changing ld does not look like a good change to me.  This tuning can be
>> placed at the compiler driver side as that is the usual place to tune
>> these things. We can ask users to specify -Wl,-Ttext-segment= (with lld
>> use --image-base= instead) .
>With the above approach we will have to target each and individual
>application/library that is statically compiled.

Sometimes the opposite. With tunable TEXT_START_ADDR you will need to
ensure every user targeting your platform use the value. It's
inconvenient for users who want to use a generic toolchain instead of a
customized one. The emerging popularity of Clang in some part is it
somewhat discourages such configure-time tuning.

>> If that is inconvenient, with Clang you can
>> add the option to a configuration file
>> (https://clang.llvm.org/docs/UsersManual.html#configuration-files).
>> With GCC there may be an option to add a fragment to the default specs
>> file.
>I'm more specifically concerned about yocto/debian builds which use
>GCC. With the proposed approach the changes are central and we can
>make sure everything will fall in place for yocto/debian rootfs builds
>with a single change.
>
>Also what I get from Palmer is that moving forward we want to adjust
>the TEXT_START_ADDR based on the page size (Huge page).

I think GCC provides a way to customize its specs file.
If 0x10000 is not a good value, I suggest just changing it to 0x200000 (lld aarch64/x86) or 0x400000 (ld.bfd aarch64/x86).
Do not provide a customization