linux-kernel - Re: [PATCH v5 1/4] riscv: Move kernel mapping to vmalloc zone

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Date:   Wed, 22 Jul 2020 14:50:42 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Palmer Dabbelt <palmer@...belt.com>
Cc:     alex@...ti.fr, paulus@...ba.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        aou@...s.berkeley.edu, Anup Patel <Anup.Patel@....com>,
        Atish Patra <Atish.Patra@....com>, zong.li@...ive.com,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-riscv@...ts.infradead.org, linux-mm@...ck.org
Subject: Re: [PATCH v5 1/4] riscv: Move kernel mapping to vmalloc zone

Benjamin Herrenschmidt <benh@...nel.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 My memory might be out of date but I *think* we do it on powerpc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


We also insert trampolines for branches, but IIUC that's a separate
issue.

cheers