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From:   Anup Patel <anup@brainfault.org>
Date:   Mon, 25 Mar 2019 18:18:45 +0530
Message-ID: <CAAhSdy2dds7pp8=XX+zbKbMoWbgjPxP6r4aMZJeHyqHV2MP4CQ@mail.gmail.com>
Subject: Re: [PATCH v3 4/4] RISC-V: Allow booting kernel from any 4KB aligned address
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Anup Patel <Anup.Patel@wdc.com>,
        Palmer Dabbelt <palmer@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Atish Patra <Atish.Patra@wdc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        "linux-riscv@lists.infradead.org" <linux-riscv@lists.infradead.org>
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On Mon, Mar 25, 2019 at 5:09 PM Christoph Hellwig <hch@infradead.org> wrote:
>
> I'm still not sold on this at all.  It is a lot more code, a lot harder
> to read code and all for a very narrow corner case that isn't even
> going to be enabled in default configs.

In case you missed my previous response about why its not just
about a very narrow corner case ......


We trying to addresses following issues in current code:
1. The current setup_vm() maps all possible kernel virtual addresses (128GB
on 64bit system and 1GB on 32bit system). The amount RAM present on
real systems might be much less so we should not have kernel mappings for
non-existent RAM. Of course, we don't know amount of RAM available in
setup_vm() so we have to split page table setup in two parts and do minimal
required mapping in setup_vm().
2. NOMMU kernel requires a swapper_pg_dir with identity mapping (VA == PA)
and without it we get boot-time crash so we cannot skip it for NOMMU case. For
NOMMU, the PAGE_OFFSET will typically be 0x80020000 (or 0x80xxxxxx). This
means swapper_pmd array (which uses -PAGE_OFFSET) will be over-sized
causing compile errors.
3. For both NOMMU with tiny memory and MMU with tiny memory, the current
setup_vm() is not allowing us to place kernel on non-2M (or non-4M) aligned
addressed there by causing memory below kernel to be wasted.
4. For MMU based kernel, the current setup_vm() is hard-wired for fixed 2M
mapping size. It will require more changes if we want to do 1G mappings.

The above issues motivated us to re-write setup_vm().

We are trying to make initial page table setup more flexible and robust so that:
1. We don't have any unwanted mappings pointing to non-existent RAM
2. We can have any value of PAGE_OFFSET for NOMMU case without the
page table arrays becoming oversized
3. We can create mappings of best possible size to get good performance
4. We can boot from any 4K/2M/1G (or just 4K) aligned load address

Also, the end result of all this is a much more readable page table setup code
shared between setup_vm() an setup_vm_final() where the differences are
abstracted via mapping ops.


Regards,
Anup