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

[Jeff Law <jeffreyalaw@gmail.com>]

On 12/18/22 15:44, Lad, Prabhakar via Binutils 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.
> 
>> 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).
Another approach here is to consider this system independent of the 
other risc-v platforms  and give it its own emulation template rather 
than polluting the generic risc-v template with this issue.

I'm leery of including these bits in the generic risc-v code given it's 
really working around, umm, quirks, mis-features or whatever we might 
want to call them.



Jeff