From: Paul Walmsley <firstname.lastname@example.org> To: Christoph Hellwig <email@example.com> Cc: 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 17:02:03 -0700 (PDT) [thread overview] Message-ID: <alpine.DEB.email@example.com> (raw) In-Reply-To: <20190802084453.GA1410@infradead.org> On Fri, 2 Aug 2019, Christoph Hellwig wrote: > 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, 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. > 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. > 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. - Paul
next prev parent reply other threads:[~2019-08-07 0:02 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 [this message] 2019-08-07 5:42 ` Christoph Hellwig 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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