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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".

  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 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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