From: Nick Desaulniers <ndesaulniers@google.com>
To: Stefan Agner <stefan@agner.ch>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>,
Nathan Chancellor <natechancellor@gmail.com>,
Arnd Bergmann <arnd@arndb.de>,
linux@armlinux.org.uk,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
LKML <linux-kernel@vger.kernel.org>,
Nicolas Pitre <nicolas.pitre@linaro.org>
Subject: Re: [PATCH 2/2] ARM: Wrap '--pic-veneer' with ld-option
Date: Wed, 5 Dec 2018 15:19:19 -0800 [thread overview]
Message-ID: <CAKwvOd=3_s9ogeHvLmb4KzAHbQWjie0rzvj8vkGjHu7HLh8hag@mail.gmail.com> (raw)
In-Reply-To: <baba21221482726e9eab67c24c308870@agner.ch>
On Wed, Dec 5, 2018 at 2:59 PM Stefan Agner <stefan@agner.ch> wrote:
>
> On 05.12.2018 19:41, Nick Desaulniers wrote:
> > On Wed, Dec 5, 2018 at 10:40 AM Ard Biesheuvel
> > <ard.biesheuvel@linaro.org> wrote:
> >>
> >> On Wed, 5 Dec 2018 at 19:36, Nathan Chancellor <natechancellor@gmail.com> wrote:
> >> >
> >> > On Wed, Dec 05, 2018 at 09:09:56AM +0100, Ard Biesheuvel wrote:
> >> > > (+ Arnd)
> >> > >
> >> > > On Wed, 5 Dec 2018 at 09:06, Nathan Chancellor <natechancellor@gmail.com> wrote:
> >> > > >
> >> > > > On Wed, Dec 05, 2018 at 08:37:05AM +0100, Ard Biesheuvel wrote:
> >> > > > > On Wed, 5 Dec 2018 at 02:42, Nathan Chancellor <natechancellor@gmail.com> wrote:
> >> > > > > >
> >> > > > > > This flag is not supported by lld:
> >> > > > > >
> >> > > > > > ld.lld: error: unknown argument: --pic-veneer
> >> > > > > >
> >> > > > > > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> >> > > > >
> >> > > > > Hi Nate,
> >> > > > >
> >> > > > > Does this mean ld.lld is guaranteed to produce position independent
> >> > > > > veneers if you build kernels that are bigger than the typical range of
> >> > > > > a relative branch?
> >> > > > >
> >> > > >
> >> > > > Hi Ard,
> >> > > >
> >> > > > Honestly, I'm not quite sure. I saw your commit that introduced this
> >> > > > flag and I wasn't quite sure what to make of it for lld. What
> >> > > > configuration would I use to verify and what would I check for?
> >> > > >
> >> > >
> >> > > Try building allyesconfig, and check the resulting binary for veneers
> >> > > (which have 'veneer' in the symbol name, at least when ld.bfd emits
> >> > > them). These veneers should not take the [virtual] address of the
> >> > > branch target directly, but take a PC relative offset (as in the
> >> > > example in the commit log of that patch you are referring to)
> >> > >
> >> >
> >> > Alright, compiling with allyesconfig is a little rough at the moment
> >> > (bug reports I will file in due time) but I was able to do it. Here's
> >> > the disassembly specifically for the functions you had in your commit,
> >> > my assembly knowledge is pretty much non-existent unfortunately so I am
> >> > not sure what to make of it (it doesn't look like there is a virtual
> >> > address for pc in that mix?). I am happy to provide any more information
> >> > that is needed.
> >> >
> >> > c03030cc <__enable_mmu>:
> >> > c03030cc: e3c00002 bic r0, r0, #2
> >> > c03030d0: e3c00b02 bic r0, r0, #2048 ; 0x800
> >> > c03030d4: e3c00a01 bic r0, r0, #4096 ; 0x1000
> >> > c03030d8: e3a05051 mov r5, #81 ; 0x51
> >> > c03030dc: ee035f10 mcr 15, 0, r5, cr3, cr0, {0}
> >> > c03030e0: ee024f10 mcr 15, 0, r4, cr2, cr0, {0}
> >> > c03030e4: eafff3c5 b c0300000 <__turn_mmu_on>
> >> > c03030e8: e320f000 nop {0}
> >> > c03030ec: e320f000 nop {0}
> >> > c03030f0: e320f000 nop {0}
> >> > c03030f4: e320f000 nop {0}
> >> > c03030f8: e320f000 nop {0}
> >> > c03030fc: e320f000 nop {0}
> >> >
> >> > c0300000 <__turn_mmu_on>:
> >> > c0300000: e1a00000 nop ; (mov r0, r0)
> >> > c0300004: ee070f95 mcr 15, 0, r0, cr7, cr5, {4}
> >> > c0300008: ee010f10 mcr 15, 0, r0, cr1, cr0, {0}
> >> > c030000c: ee103f10 mrc 15, 0, r3, cr0, cr0, {0}
> >> > c0300010: ee070f95 mcr 15, 0, r0, cr7, cr5, {4}
> >> > c0300014: e1a03003 mov r3, r3
> >> > c0300018: e1a0300d mov r3, sp
> >> > c030001c: e1a0f003 mov pc, r3
> >> >
> >>
> >> Thanks Nate.
> >>
> >> So these functions no longer appear to reside far away from each
> >> other, so there no veneer has been emitted.
> >>
> >> What we're looking for are veneers, i.e., snippets inserted by the
> >> linker that serve as a trampoline so a branch target that is far away
> >> can be reached.
> >>
> >> If no symbols exist with 'veneer' in their name *, it might make sense
> >> to rebuild the kernel as Thumb2, which has a branching range of only 8
> >> MB (as opposed to 16 MB for ARM mode)
> >
> > Heh, Arnd and I were just talking about this yesterday. Is there a
> > config that sets Thumb2 mode for the kernel?
> >
>
> Yes there is CONFIG_THUMB2_KERNEL, and it works also with LLVM/Clang.
Sounds like something we should put under CI?
https://github.com/ClangBuiltLinux/continuous-integration/issues/94
>
> However, it sometimes leads to surprising issues, like I just
> encountered a few days ago:
>
> https://lore.kernel.org/linux-pci/20181126161645.8177-1-stefan@agner.ch/
>
> --
> Stefan
>
> >>
> >> * I have no idea whether lld names its veneers like this, or even at all
--
Thanks,
~Nick Desaulniers
next prev parent reply other threads:[~2018-12-05 23:19 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-05 1:42 [PATCH 1/2] ARM: Remove '-p' from LDFLAGS Nathan Chancellor
2018-12-05 1:42 ` [PATCH 2/2] ARM: Wrap '--pic-veneer' with ld-option Nathan Chancellor
2018-12-05 7:37 ` Ard Biesheuvel
2018-12-05 8:06 ` Nathan Chancellor
2018-12-05 8:09 ` Ard Biesheuvel
2018-12-05 18:21 ` Nick Desaulniers
2018-12-05 19:30 ` Peter Smith
2018-12-05 18:36 ` Nathan Chancellor
2018-12-05 18:39 ` Ard Biesheuvel
2018-12-05 18:41 ` Nick Desaulniers
2018-12-05 22:59 ` Stefan Agner
2018-12-05 23:19 ` Nick Desaulniers [this message]
2018-12-05 19:45 ` Nathan Chancellor
2018-12-05 21:03 ` Ard Biesheuvel
2018-12-29 12:27 ` Stefan Agner
2019-01-07 19:42 ` Nick Desaulniers
2018-12-05 18:40 ` Nick Desaulniers
2018-12-05 7:37 ` [PATCH 1/2] ARM: Remove '-p' from LDFLAGS Ard Biesheuvel
2018-12-05 16:37 ` Nicolas Pitre
2018-12-05 17:47 ` Nick Desaulniers
2018-12-29 12:49 ` Stefan Agner
2018-12-31 18:12 ` [PATCH v2] " Nathan Chancellor
2019-01-05 16:34 ` Nicolas Pitre
2019-01-07 19:19 ` Nick Desaulniers
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