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

["Lad, Prabhakar" <prabhakar.csengg@gmail.com>]

Hi Palmer,

Thank you for the feedback.

On Mon, Dec 19, 2022 at 9:33 PM Palmer Dabbelt <palmer@dabbelt.com> wrote:
>
> On Mon, 19 Dec 2022 01:11:13 PST (-0800), prabhakar.csengg@gmail.com wrote:
> > Hi Jeff,
> >
> > On Mon, Dec 19, 2022 at 5:00 AM Jeff Law <jeffreyalaw@gmail.com> wrote:
> >>
> >>
> >>
> >> 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
>
> I'm never great with shell expansion, what does the "+x" do here?
>
where ${DEFAULT_TEXT_START_ADDR+x} is a parameter expansion which
evaluates to nothing if var is unset, and substitutes the string x
otherwise

> I'm also not sure how this is meant to be used/set, is there some
> autoconf changes that this needs to come along with so this can be set?
>
There is no autoconf for this, so when trying to build with a
different offset we just export DEFAULT_TEXT_START_ADDR in the shell
with a value and then build it. So for example when trying to build
from Yocto we just bbappend the binutils file and export the
DEFAULT_TEXT_START_ADDR variable.

> >> >>> +    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.
> >>
> > Sounds good to me, For example it would include the RISC-V generic
> > template and we just override the offset in the platform specific conf
> > file. Having said that, I'm not sure of the maintainability of such
> > files and RISC-V maintainers will be OK for such change.
>
> I was assuming we'd have a configure option somewhere in the toolchain
> to control the default.  I hadn't really thought of where, either
> binutils or GCC could make sense (as it's driven by a `-Wl` option).  I
> haven't quite sorted out how this modification to ldparams does that (as
> above).
>
> That said, maybe the right answer here is actually to just have
> "-mcpu=renesas-rzfve" just include -Ttext-segement= in LINK_SPEC?
So that means we want GCC to accept  "renesas-rzfive" as an mcpu
option and then we update LINK_SPEC [0] based on the mcpu parameter.

[0] https://github.com/gcc-mirror/gcc/blob/releases/gcc-9/gcc/config/riscv/linux.h#L60

Is my understanding correct here?

> if we implement the Linux side of this as "don't mmap() unless the hint says
> to, and then context switch the memory" then it's essentially just a CPU
> tuning parameter (with a giant security hole, but nothing we can do
> about that).
>
I'm not sure how maintainers will react to this if we propose a
solution. I wonder if we are just alone with such a problem.

> > I'm curious what Palmer has in his mind on tuning this parameter for
> > huge pages. If that exposes some sort of config option we could use
> > that option to set the offset.
>
> The hugepage thing might not be all that exiciting: it's just a question
> of whether we're defaulting to 2nd-level or 3rd-level paging.  Having
> the default as-is for 2nd-level paging seems reasonable as it gives a
> lot more range for symbols, so maybe nobody even wants to default to the
> 3rd-level offsets.
>
Okay, so I won't play the huge page card ;)

Cheers,
Prabhakar