Messages in this thread | | | Date | Tue, 27 Aug 2019 07:46:24 -0700 | From | Christoph Hellwig <> | Subject | Re: [RFC PATCH 0/2] Add support for SBI version to 0.2 |
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On Mon, Aug 26, 2019 at 04:32:54PM -0700, Atish Patra wrote: > This patch series aims to add support for SBI specification version > v0.2. It doesn't break compatibility with any v0.1 implementation. > Internally, all the v0.1 calls are just renamed to legacy to be in > sync with specification [1]. > > The patches for v0.2 support in OpenSBI are available at > http://lists.infradead.org/pipermail/opensbi/2019-August/000422.html > > [1] https://github.com/riscv/riscv-sbi-doc/blob/master/riscv-sbi.adoc
I really don't like the current design of that SBI 0.2 spec, and don't think implementing it as-is is helpful.
For one the way how the extension id is placed creates a compatibilty problem, not allowing your to implement a backwards compatible sbi, which seems bad.
Second just blindly moving all the existing calls to a single legacy extension doesn't seem useful. We need to differenciate the existing calls:
(1) actually board specific and have not place in a cpu abstraction layer: getchar/putchar, these should just never be advertised in a non-legacy setup, and the drivers using them should not probe on a sbi 0.2+ system (2) useful for currently taped out cpus and in the long run for virtualization to avoid mmio traps: ipis, timers, tlb shootdown. These should stay backwards compatible, but for sbi 0.2 be negotiated individually (3) in theory useful, but given how much of a big hammer sfence.i not useful in theory: SBI_REMOTE_FENCE_I we can decide if we want to either not allow it for sbi 0.2+ or also negotiate it. I'd personally favor not advertising it and just use ipis to implement it. If we want useful acceleration of i-cache synchronization we'll need actual instructions that are much more fine grained in the future.
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