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Date:   Wed, 27 Mar 2019 00:54:41 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Anup Patel <anup@brainfault.org>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Palmer Dabbelt <palmer@sifive.com>,
        Anup Patel <Anup.Patel@wdc.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Atish Patra <Atish.Patra@wdc.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        "linux-riscv@lists.infradead.org" <linux-riscv@lists.infradead.org>
Subject: Re: [PATCH v3 4/4] RISC-V: Allow booting kernel from any 4KB aligned
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On Mon, Mar 25, 2019 at 09:46:59PM +0530, Anup Patel wrote:
> > Why do you even care about kernel mappings for non-existant ram.
> 
> We care because there will always be some buggy kernel driver/code going
> out-of-bound and accessing non-existent RAM. If we by default map all
> possible kernel virtual address then behaviour of buggy accesses will be
> unpredictable.
> 
> Further, I think we should also make .text and .rodata sections of kernel
> as read-only. This will protect kernel code and rodata.

All of that is useful at the final_setup_vm() time - but none of it
matters during early setup_vm where life is complicated.

Mike suggested on the previous iteration that you only do smaller
mappings when setting up the final mapping to avoid the ops churn,
and I fully agree with him. 

So I would suggest we avoid complicated the fiddly early boot changes
that just add complxity, and you instead redirect your efforts to
say implemented proper ro and non-executable sections using 4k mappings
in the final VM setup only.  That should actuall lead to less code
and complexity, and provide more benefits.