Messages in this thread | | | From | Alexandre Ghiti <> | Date | Tue, 28 Sep 2021 16:58:51 +0200 | Subject | Re: [PATCH V2 1/2] riscv: Add RISC-V svpbmt extension |
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On Tue, Sep 28, 2021 at 3:48 PM Philipp Tomsich <philipp.tomsich@vrull.eu> wrote: > > Nick, > > On Tue, 28 Sept 2021 at 15:19, Nick Kossifidis <mick@ics.forth.gr> wrote: > > > > On 9/28/21 7:26 AM, Atish Patra wrote: > > > On Mon, Sep 27, 2021 at 8:50 PM Anup Patel <anup@brainfault.org> wrote: > > >> > > >> On Tue, Sep 28, 2021 at 6:32 AM Nick Kossifidis <mick@ics.forth.gr> wrote: > > >>> > > >>> Στις 2021-09-27 23:13, Atish Patra έγραψε: > > >>>>> We need to decide whether we should support the upstream kernel for > > >>>>> D1. Few things to consider. > > >>>>> – Can it be considered as an errata ? > > >>> > > >>> It's one thing to follow the spec and have an error in the > > >>> implementation, and another to not follow the spec. > > >>> > > >>>>> – Does it set a bad precedent and open can of worms in future ? > > >>> > > >>> IMHO yes, I'm thinking of Kendryte 210 devs for example coming up and > > >>> asking for MMU support, they 've also shipped many chips already. I can > > >>> also imagine other vendors in the future coming up with implementations > > >>> that violate the spec in which case handling the standard stuff will > > >>> become messy and complex, and hurt performance/security. We'll end up > > >>> filling the code with exceptions and tweaks all over the place. We need > > >>> to be strict about what is "riscv" and what's "draft riscv" or "riscv > > >>> inspired", and what we are willing to support upstream. I can understand > > >>> supporting vendor extensions upstream but they need to fit within the > > >>> standard spec, we can't have for example extensions that use encoding > > >>> space/csrs/fields etc reserved for standard use, they may only use > > >>> what's reserved for custom/vendor use. At least let's agree on that. > > >> > > >> Totally agree with Nick here. It's a slippery slope. > > >> > > >> Including D1 PTE bits (or Kendryte K210 MMU) part of the Linux RISC-V > > >> means future hardware which intentionally violates specs will also have to > > >> be merged and the RISC-V patch acceptance policy will have no significance. > > >> > > >>> > > >>>>> – Can we just ignore D1 given the mass volume ? > > >>>>> > > >>> > > >>> IMHO no, we need to find a way to support it upstream but I believe > > >>> there is another question to answer: > > >>> > > >>> Do we also guarantee "one image to rule them all" approach, required by > > >>> binary distros, for implementations that violate the spec ? Are we ok > > >>> for example to support Allwinner D1 upstream but require a custom > > >>> configuration/build instead of supporting it with the "generic" image ? > > >>> In one case we need to handle the violation at runtime and introduce > > >>> overhead for everyone (like looking up __riscv_svpbmt every time we set > > >>> a PTE in this case), in the other it's an #ifdef. > > >> > > >> At least, we should not have hardware violating specs as part of the > > >> unified kernel image instead have these intentional deviations/violations > > >> under separate kconfig which will not be enabled by default. This means > > >> vendors (of such hardware) and distros will have to explicitly enable > > >> support for such violations/deviations. > > >> > > > > > > If we merge the code and are not enabled by default, it would be a > > > maintenance nightmare in future. > > > These part of the kernel will not be regularly tested but we have to > > > carry the changes for a long time. > > > > I don't see a difference between having these features as part of the > > generic image vs having them as custom configs/builds. The code will get > > executed only on boards that support the custom/non-compliant > > implementation anyway. To the contrary we'll have more code to test if > > we are doing things at runtime vs at compile time. > > > > > Similar changes will only grow over time causing a lot of custom > > > configs that are not enabled by default. > > > > > > > We'll have a lot of custom configs that will only get used on boards > > that use them, vs runtime code that will run for no reason on every > > board and choose the default/standard-compliant implementation most of > > the time. In the end the code will only get tested on specific hardware > > anyway. > > > > > IMHO, if we want to support this board in upstream, we should just > > > clearly state that it is one time special exception > > > for this board only because of the following reasons > > > > > > 1. The board design predates the patch acceptance policy. > > > 2. We don't have enough affordable Linux compatible platforms today. > > > 3. Allowing running an upstream kernel on D1 helps the RISC-V software > > > ecosystem to grow. > > > > > > > The same can be said for Kendryte as well, are we willing to also > > support their MMU implementation on the generic image if a patch comes > > in? To be clear I'm not saying we shouldn't support D1 or Kendryte > > upstream, I'm just saying that we shouldn't sacrifice the complexity and > > performance of the code path for standard-compliant implementations, to > > support non-compliant implementations, and instead support non-compliant > > implementations with custom kernel builds using compile time options. It > > For priming the pump on the software effort, having a solution that is enabled > on distro-builds is clearly preferable — that leads to the solution that Palmer > had outlined at LPC, which is to have a KCONFIG option that enables the > alternate code paths and can be turned off for embedded use-cases. > > > still counts as upstream support, they won't have to maintain their own > > forks. It'll also allow custom implementations to have more flexibility > > on what they can do since they will be able to use completely > > different/custom code paths, instead of trying to fit in the standard > > code path (which will become a mess over time). I think this approach is > > much more flexible and will allow more customizations to be supported > > upstream in the future. > > The important detail will be the ground rules: changes have to be sufficiently > quarantined that (a) they can be turned off, (b) can be reverted easily (in case > that vendors fail to perform their maintenance obligations),
Can we really remove support once it is in and widely used?
> and (c) they don't > affect the performance and complexity of the standard code paths. > > Cheers, > Philipp. > > _______________________________________________ > linux-riscv mailing list > linux-riscv@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-riscv
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