From: Palmer Dabbelt <palmer@dabbelt.com>
To: mpe@ellerman.id.au
Cc: benh@kernel.crashing.org, alex@ghiti.fr, paulus@samba.org,
Paul Walmsley <paul.walmsley@sifive.com>,
aou@eecs.berkeley.edu, Anup Patel <Anup.Patel@wdc.com>,
Atish Patra <Atish.Patra@wdc.com>,
zong.li@sifive.com, linux-kernel@vger.kernel.org,
linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
linux-mm@kvack.org
Subject: Re: [PATCH v5 1/4] riscv: Move kernel mapping to vmalloc zone
Date: Tue, 21 Jul 2020 22:46:38 -0700 (PDT) [thread overview]
Message-ID: <mhng-378c6e4e-9eba-4bfd-89d6-b4d2549ff3a1@palmerdabbelt-glaptop1> (raw)
In-Reply-To: <87sgdkqhjx.fsf@mpe.ellerman.id.au>
On Tue, 21 Jul 2020 21:50:42 PDT (-0700), mpe@ellerman.id.au wrote:
> Benjamin Herrenschmidt <benh@kernel.crashing.org> writes:
>> On Tue, 2020-07-21 at 16:48 -0700, Palmer Dabbelt wrote:
>>> > Why ? Branch distance limits ? You can't use trampolines ?
>>>
>>> Nothing fundamental, it's just that we don't have a large code model in the C
>>> compiler. As a result all the global symbols are resolved as 32-bit
>>> PC-relative accesses. We could fix this with a fast large code model, but then
>>> the kernel would need to relax global symbol references in modules and we don't
>>> even do that for the simple code models we have now. FWIW, some of the
>>> proposed large code models are essentially just split-PLT/GOT and therefor
>>> don't require relaxation, but at that point we're essentially PIC until we
>>> have more that 2GiB of kernel text -- and even then, we keep all the
>>> performance issues.
>>
>> My memory might be out of date but I *think* we do it on powerpc
>> without going to a large code model, but just having the in-kernel
>> linker insert trampolines.
>
> We build modules with the large code model, and always have AFAIK:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/Makefile?commit=4fa640dc52302b5e62b01b05c755b055549633ae#n129
>
> # -mcmodel=medium breaks modules because it uses 32bit offsets from
> # the TOC pointer to create pointers where possible. Pointers into the
> # percpu data area are created by this method.
> #
> # The kernel module loader relocates the percpu data section from the
> # original location (starting with 0xd...) to somewhere in the base
> # kernel percpu data space (starting with 0xc...). We need a full
> # 64bit relocation for this to work, hence -mcmodel=large.
> KBUILD_CFLAGS_MODULE += -mcmodel=large
Well, a fast large code model would solve a lot of problems :). Unfortunately
we just don't have enough people working on this stuff to do that. It's a
somewhat tricky thing to do on RISC-V as there aren't any quick sequences for
long addresses, but I don't think we're that much worse off than everyone else.
At some point I had a bunch of designs written up, but they probably went along
with my SiFive computer. I think we ended up decided that the best bet would
be to distribute constant tables throughout the text such that they're
accessible via the 32-bit PC-relative loads at any point -- essentially the
multi-GOT stuff that MIPS used for big objects. Doing that well is a lot of
work and doing it poorly is just as slow as PIC, so we never got around to it.
> We also insert trampolines for branches, but IIUC that's a separate
> issue.
"PowerPC branch trampolines" points me here
https://sourceware.org/binutils/docs-2.20/ld/PowerPC-ELF32.html . That sounds
like what we're doing already in the medium code models: we have short and
medium control transfer sequences, linker relaxation optimizes them when
possible. Since we rely on linker relaxation pretty heavily we just don't
bother with the smaller code model: it'd be a 12-bit address space for data and
a 21-bit address space for text (with 13-bit maximum function size). Instead
of building out such a small code model we just spent time improving the linker.
next prev parent reply other threads:[~2020-07-22 5:46 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-07 7:59 [PATCH v5 0/4] vmalloc kernel mapping and relocatable kernel Alexandre Ghiti
2020-06-07 7:59 ` [PATCH v5 1/4] riscv: Move kernel mapping to vmalloc zone Alexandre Ghiti
2020-06-11 21:34 ` Atish Patra
2020-06-12 12:30 ` Alex Ghiti
2020-07-09 5:05 ` Palmer Dabbelt
2020-07-09 8:15 ` Zong Li
2020-07-09 11:11 ` Alex Ghiti
2020-07-21 18:36 ` Alex Ghiti
2020-07-21 19:05 ` Palmer Dabbelt
2020-07-21 23:12 ` Benjamin Herrenschmidt
2020-07-21 23:48 ` Palmer Dabbelt
2020-07-22 2:21 ` Benjamin Herrenschmidt
2020-07-22 4:50 ` Michael Ellerman
2020-07-22 5:46 ` Palmer Dabbelt [this message]
2020-07-22 9:43 ` Arnd Bergmann
2020-07-22 19:52 ` Palmer Dabbelt
2020-07-22 20:22 ` Arnd Bergmann
2020-07-22 21:05 ` Atish Patra
2020-07-24 7:20 ` Arnd Bergmann
2020-07-23 5:32 ` Alex Ghiti
2020-07-21 23:11 ` Benjamin Herrenschmidt
2020-07-21 23:36 ` Palmer Dabbelt
2020-07-23 5:36 ` Alex Ghiti
2020-07-23 5:21 ` Alex Ghiti
2020-07-23 22:33 ` Benjamin Herrenschmidt
2020-07-24 8:14 ` Arnd Bergmann
2020-06-07 7:59 ` [PATCH v5 2/4] riscv: Introduce CONFIG_RELOCATABLE Alexandre Ghiti
2020-06-10 14:10 ` Jerome Forissier
2020-06-11 19:43 ` Alex Ghiti
2020-06-07 7:59 ` [PATCH v5 3/4] powerpc: Move script to check relocations at compile time in scripts/ Alexandre Ghiti
2020-06-07 7:59 ` [PATCH v5 4/4] riscv: Check relocations at compile time Alexandre Ghiti
2020-07-08 4:21 ` [PATCH v5 0/4] vmalloc kernel mapping and relocatable kernel Alex Ghiti
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