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

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Date:   Wed, 22 Jul 2020 12:21:16 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Palmer Dabbelt <palmer@...belt.com>
Cc:     alex@...ti.fr, mpe@...erman.id.au, 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

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My memory might be out of date but I *think* we do it on powerpc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.

Cheers,
Ben.