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Date:   Mon, 18 Mar 2019 09:12:55 -0700 (PDT)
From:   Paul Walmsley <paul.walmsley@sifive.com>
To:     Mike Rapoport <rppt@linux.ibm.com>
cc:     Anup Patel <anup@brainfault.org>, Anup Patel <Anup.Patel@wdc.com>,
        Palmer Dabbelt <palmer@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Atish Patra <Atish.Patra@wdc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Christoph Hellwig <hch@infradead.org>,
        "linux-riscv@lists.infradead.org" <linux-riscv@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/3] RISC-V: Allow booting kernel from any 4KB aligned
 address
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On Mon, 18 Mar 2019, Mike Rapoport wrote:

> On Sat, Mar 16, 2019 at 04:55:30AM +0530, Anup Patel wrote:
> 
> > We started exploring ways to re-claim memory below kernel because
> > we are trying to get Linux working on Kendryte K210 board
> > (https://kendryte.com/). This board has dual-core 64bit RISC-V but it
> > only has 8MB RAM.
> 
> Huh, 8MB of RAM is tough...
> 
> It is possible to use the memory below the kernel, e.g x86-64 does that.
> But it is definitely a separate change and with such RAM diet using 4K
> pages seems unavoidable.
> 
> I still have concern about using 4K pages whenever the load address is not
> 2M (4M) aligned. People tend to not pay enough attention to such details
> and they would load the kernel at an arbitrary address and get the
> performance hit.
> 
> I think the default should remain as is and the ability to map the kernel
> with 4K pages (and use 4K aligned load address) should be a Kconfig option.

Agreed.  That Kconfig parameter should also be default-off.
  
Only a small number of people will try to run RISC-V Linux on a Kendryte 
board.  That niche use-case shouldn't impact the much larger group of 
people who will run Linux on more reasonably-sized systems.  No one should 
need to ask people to report their kernel load address whenever someone
reports a performance regression.


- Paul