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SubjectRe: [PATCH V2 1/2] riscv: Add RISC-V svpbmt extension
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.

>> – 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.

Regards,
Nick

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