Re: [PATCH 2/4] riscv: Fix loading 64-bit NOMMU kernels past the start of RAM

From: Conor Dooley <conor@kernel.org>
To: Samuel Holland <samuel.holland@sifive.com>
Cc: Conor Dooley <conor.dooley@microchip.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/4] riscv: Fix loading 64-bit NOMMU kernels past the start of RAM
Date: Wed, 28 Feb 2024 15:37:30 +0000	[thread overview]
Message-ID: <20240228-cavity-aground-d1e8072a2c0d@spud> (raw)
In-Reply-To: <9673cbe7-41ec-4261-9be8-fc5191ade56c@sifive.com>

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On Tue, Feb 27, 2024 at 01:22:12PM -0600, Samuel Holland wrote:
> Hi Conor,
> 
> On 2024-02-27 6:18 AM, Conor Dooley wrote:
> > On Mon, Feb 26, 2024 at 04:34:47PM -0800, Samuel Holland wrote:
> >> commit 3335068f8721 ("riscv: Use PUD/P4D/PGD pages for the linear
> >> mapping") added logic to allow using RAM below the kernel load address.
> >> However, this does not work for NOMMU, where PAGE_OFFSET is fixed to the
> >> kernel load address. Since that range of memory corresponds to PFNs
> >> below ARCH_PFN_OFFSET, mm initialization runs off the beginning of
> >> mem_map and corrupts adjacent kernel memory. Fix this by restoring the
> >> previous behavior for NOMMU kernels.
> >>
> >> Fixes: 3335068f8721 ("riscv: Use PUD/P4D/PGD pages for the linear mapping")
> > 
> > This commit was a year ago, why has nobody reported this as being an
> > issue before?
> 
> I can think of a few reasons:
> 1) NOMMU users are likely to be using RV32, which is not affected.
> 2) Before patch 4 of this series, NOMMU implied M-mode, so there was nothing in
> the way to prevent loading Linux at the very beginning of RAM. (U-Boot/SPL
> relocates itself to the end of RAM, so it would not cause a problem.)
> 3) Platforms where RAM does not begin at exactly 0x80000000 would be affected,
> there are several workarounds: change the start of RAM (for soft cores), change
> PAGE_OFFSET, or change the memory ranges in the devicetree to exclude anything
> below PAGE_OFFSET.
> 
> It's possible that nobody was affected, but it's still technically a regression
> (a hypothetical platform with RAM from 0x40000000 to 0xc0000000 would crash
> instead of only being able to use half its RAM), so I thought it still deserved
> the Fixes: tag.

Right, thanks for explaining.

Cheers,
Conor.

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