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    SubjectRe: [RFC PATCH 0/2] Add support for SBI version to 0.2
    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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