linux-kernel - Re: [PATCH v2 3/5] RISC-V: Allow booting kernel from any 4KB aligned address

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Message-ID: <20190323172421.GA14080@infradead.org>
Date:   Sat, 23 Mar 2019 10:24:21 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     Anup Patel <Anup.Patel@....com>, Albert Ou <aou@...s.berkeley.edu>,
        Palmer Dabbelt <palmer@...ive.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Atish Patra <Atish.Patra@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH v2 3/5] RISC-V: Allow booting kernel from any 4KB aligned
 address

On Sat, Mar 23, 2019 at 05:40:12PM +0200, Mike Rapoport wrote:
> I have no general objection, but I presume the patch will be significantly
> simplified if the addition of 4K pages support will follow the removal of
> the trampoline_pd_dir. 
> 
> That said, I didn't look into the details, since they will change
> substantially, only some comments on the Kconfig part.
> 
> On the high level, have you considered using large pages in setup_vm() and
> the remapping everything with 4K pages in setup_vm_final()?  This might
> save you the whole ops-> churn.

That would be a great start.  That being said the current tiny memory
RISC-V devices don't even have a MMU, so the kernel pagetable mapping
isn't even relevant for them.  I'm just not sure adding more complexity
in the early boot path for a borderline case (MMU and tiny memory
with a tiny kernel image) is reall worth all the complexity.