From 338afe770f65ca071252f6c0f919624cbbf94062 Mon Sep 17 00:00:00 2001 From: Luke Kenneth Casson Leighton Date: Thu, 26 Apr 2018 08:43:24 +0100 Subject: [PATCH] start filling in --- isa_conflict_resolution.mdwn | 34 ++++++++++++++++++++++++++++++++++ 1 file changed, 34 insertions(+) diff --git a/isa_conflict_resolution.mdwn b/isa_conflict_resolution.mdwn index b2b3b2124..d72c0a54e 100644 --- a/isa_conflict_resolution.mdwn +++ b/isa_conflict_resolution.mdwn @@ -158,6 +158,15 @@ be hardest hit by the resultant chaos, and that will just be the more TBD (basically, may not be RV Foundation's "scope", still results in problem, so not an option) +The summary here was that Compliance testing of Custom Extensions is +not just out-of-scope, but even if it was taken into account that +binary-encoding meanings could change, it would still be out-of-scope. + +However at the time that this argument was made, it had not yet been +appreciated fully the impact that revisions to the Standard would have, +when billions of dollars worth of (older, legacy) RISC-V hardware had +already been deployed. + Two interestingly diametrically-opposed equally valid arguments exist here: * Whilst Compliance testing of Custom Extensions is definitely legitimately @@ -197,6 +206,31 @@ a hundred percent unsuitable for solving the problem. TBD, basically same as mvend/march WARL except needs an extra CSR where mv/ma doesn't. +Out of the MISA discussion came a "MISA-like" proposal, which would +take into account the flaws pointed out by trying to use "MISA": + +* The MISA-like CSR's meaning would be identified by compilers using the + mvendor-id/march-id tuple as a compiler target +* Each custom-defined bit of the MISA-like CSR would (mutually-exclusively) + redirect binary encoding(s) to specific encodings +* No Extension would *actually* be disabled: its internal state would + be left on (permanently) so that switching could be done inside + inner loops. + +Whilst it was the first "workable" solution it was also noted that the +scheme is quite invasive: it requires an entirely new CSR to be added +to the privileged spec. This does not completely fulfil the "minimum +impact" requirement. + +Also interesting around the same time an additional discussion was +raised that covered the *compiler* side of the same equation. This +revolved around using mvendorid-marchid tuples at the compiler level, +to be put into assembly output (by gcc), preserving the required +*globally* unique identifying information for binutils to successfully +turn the custom instruction into an actual binary-encoding (plus +binary-encoding of the context-switching information). (**TBD, Jacob, +separate page? review this para?**) + # mvendorid/marchid WARL TBD paraphrase and clarify -- 2.30.2