linux-kernel - Re: [PATCH] riscv: kbuild: add virtual memory system selection

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Message-ID: <20190802084453.GA1410@infradead.org>
Date:   Fri, 2 Aug 2019 01:44:53 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Paul Walmsley <paul.walmsley@...ive.com>
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Alexandre Ghiti <alex@...ti.fr>
Subject: Re: [PATCH] riscv: kbuild: add virtual memory system selection

On Fri, Jul 26, 2019 at 01:00:49PM -0700, Paul Walmsley wrote:
> 
> The RISC-V specifications currently define three virtual memory
> translation systems: Sv32, Sv39, and Sv48.  Sv32 is currently specific
> to 32-bit systems; Sv39 and Sv48 are currently specific to 64-bit
> systems.  The current kernel only supports Sv32 and Sv39, but we'd
> like to start preparing for Sv48.  As an initial step, allow the
> virtual memory translation system to be selected via kbuild, and stop
> the build if an option is selected that the kernel doen't currently
> support.
> 
> This patch currently has no functional impact.

It cause the user to be able to select a config which thus won't build.
So it is not just useless, which already is a reason not to merge it,
but actively harmful, which is even worse.

Even if we assume we want to implement Sv48 eventually (which seems
to be a bit off), we need to make this a runtime choice and not a
compile time one to not balloon the number of configs that distributions
(and kernel developers) need to support.