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

[Lad, Prabhakar]

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.

> 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).

Cheers,
Prabhakar