From: Christoph Hellwig <email@example.com> To: Paul Walmsley <firstname.lastname@example.org> Cc: Christoph Hellwig <email@example.com>, firstname.lastname@example.org, email@example.com, Alexandre Ghiti <firstname.lastname@example.org> 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.email@example.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".
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 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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