linux-kernel - Re: [PATCH v2] riscv: hwprobe: export largest userspace address

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Date: Wed, 21 Feb 2024 14:51:46 +0000
From: Conor Dooley <conor@...nel.org>
To: Clément Léger <cleger@...osinc.com>
Cc: Jonathan Corbet <corbet@....net>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, linux-doc@...r.kernel.org,
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Robin Ehn <rehn@...osinc.com>,
	Charlie Jenkins <charlie@...osinc.com>,
	Stefan O'Rear <sorear@...tmail.com>,
	Jessica Clarke <jrtc27@...c27.com>
Subject: Re: [PATCH v2] riscv: hwprobe: export largest userspace address

On Tue, Feb 20, 2024 at 12:09:46PM +0100, Clément Léger wrote:
> Some userspace applications (OpenJDK for instance) uses the free MSBs
> in pointers to insert additional information for their own logic and
> need to get this information from somewhere. Currently they rely on
> parsing /proc/cpuinfo "mmu=svxx" string to obtain the current value of
> virtual address used bits [1]. Since this reflect the raw MMU mode
> supported, it might differ from the logical one used internally.
> Exporting the maximum mmappable address through hwprobe will allow a
> more stable interface to be used. For that purpose, add a new hwprobe
> key named RISCV_HWPROBE_MAX_KEY which will export the maximum mmappable
> userspace address.
> 
> Link: https://github.com/openjdk/jdk/blob/master/src/hotspot/os_cpu/linux_riscv/vm_version_linux_riscv.cpp#L171 [1]
> Signed-off-by: Clément Léger <cleger@...osinc.com>

Doesn't build for nommu:
https://patchwork.kernel.org/project/linux-riscv/patch/20240220110950.871307-1-cleger@rivosinc.com/

Cheers,
Conor.

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