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Subject: Re: [PATCH v5 1/4] riscv: Move kernel mapping to vmalloc zone
From:   Benjamin Herrenschmidt <benh@kernel.crashing.org>
To:     Palmer Dabbelt <palmer@dabbelt.com>
Cc:     alex@ghiti.fr, mpe@ellerman.id.au, 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
Date:   Wed, 22 Jul 2020 12:21:16 +1000
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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My memory might be out of date but I *think* we do it on powerpc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.