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

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Date:   Tue, 6 Aug 2019 22:42:46 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Paul Walmsley <paul.walmsley@...ive.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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 Tue, Aug 06, 2019 at 05:02:03PM -0700, Paul Walmsley wrote:
> The rationale is to encourage others to start laying the groundwork for 
> future Sv48 support.  The immediate trigger for it was Alex's mmap 
> randomization support patch series, which needs to set some Kconfig 
> options differently depending on the selection of Sv32/39/48.  

Writing a formal todo list is much better encouragement than adding
dead code.  Th latter has a tendency of lingering around forever and
actually hurting people.

> 
> > but actively harmful, which is even worse.
> 
> Reflecting on this assertion, the only case that I could come up with is 
> that randconfig or allyesconfig build testing could fail.  Is this the 
> case that you're thinking of, or is there a different one?  If that's the 
> one, I do agree that it would be best to avoid this case, and it looks 
> like there's no obvious way to work around that issue.

randconfig or just a user thinking bigger is better and picking it.

> > 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.
> 
> The expectation is that kernels that support multiple virtual memory 
> system modes at runtime will probably incur either a performance or a 
> memory layout penalty for doing so.  So performance-sensitive embedded 
> applications will select only the model that they use, while distribution 
> kernels will likely take the performance hit for broader single-kernel 
> support.

Even if we want to support Sv39 only or Sv39+Sv39 the choice in the
patch doesn't make any sense.  So better do the whole thing when its
ready than doing false "groundwork".