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From: Nelson Chu <nelson@rivosinc.com>
To: Palmer Dabbelt <palmer@dabbelt.com>
Cc: prabhakar.csengg@gmail.com, jeffreyalaw@gmail.com, i@maskray.me,
	 prabhakar.mahadev-lad.rj@bp.renesas.com,
	Andrew Waterman <andrew@sifive.com>,
	 Jim Wilson <jim.wilson.gcc@gmail.com>,
	binutils@sourceware.org,  Chris.Paterson2@renesas.com
Subject: Re: [RFC PATCH] ld/emulparams: elf32lriscv-defs: Add support tune the text segment start address
Date: Wed, 21 Dec 2022 21:24:19 +0800	[thread overview]
Message-ID: <CAPpQWtD0djcuCJhqS9LibgOJx-t0TapJzSKr9i+rPoajOXT=_g@mail.gmail.com> (raw)
In-Reply-To: <mhng-418288c2-31af-4c71-9978-aa44a6322d4f@palmer-ri-x1c9a>

On Tue, Dec 20, 2022 at 5:33 AM 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?

I think it should be similar to what nds32 did in the
ld/emulparams/nds32elf_linux.sh, but I'm not really sure how it works.
When should the DEFAULT_TEXT_START_ADDR be defined?  Maybe when
configure or make, not sure about that.  But at least I only see the
changes in nds32, all of the other targets won't do that, so this must
be a handy little trick, but there are always other ways to do the
same things - for example, what maskray said should be the general way
to do.  I don't have any opinion on this change, if it won't break the
general build, then probably acceptable.  Just that it seems need to
introduce the DEFAULT_TEXT_START_ADDR or whatever configure options,
so this isn't only be the riscv gnu ld's issue, this will also be the
issue of lld, so if the maintainers of lld already have some
suggestions, then it will definitely worth to referring to.

Thanks
Nelson

> 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?
>
> >> >>> +    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?  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 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.
>
> >> 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.
> >>
> > :-)
>
> I think we have way bigger pollution problems than this.
>
> >
> > Cheers,
> > Prabhakar

  reply	other threads:[~2022-12-21 13:24 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-12-16 21:43 Lad Prabhakar
2022-12-17  9:19 ` Andrew Waterman
2022-12-18  6:24 ` Fangrui Song
2022-12-18 22:44   ` Lad, Prabhakar
2022-12-18 23:09     ` Fangrui Song
2022-12-19  9:02       ` Lad, Prabhakar
2022-12-19  5:00     ` Jeff Law
2022-12-19  9:11       ` Lad, Prabhakar
2022-12-19 21:33         ` Palmer Dabbelt
2022-12-21 13:24           ` Nelson Chu [this message]
2022-12-21 20:15             ` Lad, Prabhakar
2022-12-22  2:12               ` Nelson Chu
2022-12-21 20:06           ` Lad, Prabhakar

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