From: Christoph Hellwig <hch@infradead.org>
To: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Christoph Hellwig <hch@infradead.org>,
linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
Alexandre Ghiti <alex@ghiti.fr>
Subject: Re: [PATCH] riscv: kbuild: add virtual memory system selection
Date: Tue, 6 Aug 2019 22:42:46 -0700 [thread overview]
Message-ID: <20190807054246.GB1398@infradead.org> (raw)
In-Reply-To: <alpine.DEB.2.21.9999.1908061648220.13971@viisi.sifive.com>
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".
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next prev parent reply other threads:[~2019-08-07 5:42 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-26 20:00 [PATCH] riscv: kbuild: add virtual memory system selection Paul Walmsley
2019-07-28 13:38 ` Bin Meng
2019-07-31 19:37 ` Paul Walmsley
2019-08-01 8:56 ` Bin Meng
2019-08-02 8:44 ` Christoph Hellwig
2019-08-07 0:02 ` Paul Walmsley
2019-08-07 5:42 ` Christoph Hellwig [this message]
2019-08-07 7:04 ` Alexandre Ghiti
2019-08-07 15:12 ` Christoph Hellwig
2019-08-07 16:20 ` Palmer Dabbelt
2019-08-07 16:42 ` Paul Walmsley
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