Specify misa.M behaviour with Zmmul

[Timmmm]

From this comment:

misa spec requires that if M is set, then M is available. If only Zmmul is present misa.M should be clear. If misa.M is writable and M is cleared, the specs say that the hart behaves as if M not present, where behavior includes the opcodes being "reserved". Whether Zmmul is still usable when misa.M is clear is implementation-defined; misa allows the behaviors to be "reserved" in this case - so Zmmul if still available, it acts as a standard extension. This I believe is the only way to have a consistent view of misa behavior matching the ratified specifications.

To avoid spec-by-github-issue please can we add that information to the actual spec? It is not obvious.

I think it is correct though since it matches the B extension and its sub-extensions. The C extension/sub-extensions work differently.

Ideally a whole section would be added to misa explaining how the sub-extensions are supposed to work for M, B, C, etc.

[allenjbaum]

Originally, I wrote the Zmmul spec to say Zmmul was incompatible with M - but that was rejected because it was pointed out that if misa.M was writable, and Zmmul was implemented, then the effect would be to disable divide/remainder (whose opcodes become reserved), and that should be legal. There was no discussion that it would have different behavior when misa.M was writable or not, just what its value was. (There was also no discussion of why anyone would want to dynamically disable just divide instructions, leaving multiply enabled. The interpretation that multiply ops are implementation defined in the case where Zmmul is implemented, and misa.M is writable and clear- sees inconsistent. The meaning shouldn't change depending on how the misa.M bit was configured, just what its value is. (It gets more interesting since it is legal for all of misa to be RdOnly0) *Zmmul Misa.M meaning* 0 RO1 mul/div ops are legal 0 RO0 mul/div ops are reserved 0 RW0 mul/div ops are reserved 1 RO1 mul/div ops are legal 1 RO0 mul ops are legal, div/rem ops are reserved 1 RW0 mul ops are legal, div/rem ops are reserved

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On Tue, Nov 5, 2024 at 8:06 AM Tim Hutt ***@***.***> wrote: From this comment <#1121 (comment)> : misa spec requires that if M is set, then M is available. If only Zmmul is present misa.M should be clear. If misa.M is writable and M is cleared, the specs say that the hart behaves as if M not present, where behavior includes the opcodes being "reserved". Whether Zmmul is still usable when misa.M is clear is implementation-defined; misa allows the behaviors to be "reserved" in this case - so Zmmul if still available, it acts as a standard extension. This I believe is the only way to have a consistent view of misa behavior matching the ratified specifications. To avoid spec-by-github-issue please can we add that information to the actual spec? It is not obvious. I think it is correct though since it matches the B extension and its sub-extensions. The C extension/sub-extensions work differently. Ideally a whole section would be added to misa explaining how the sub-extensions are supposed to work for M, B, C, etc. — Reply to this email directly, view it on GitHub <#1711>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AHPXVJX2YCMJFJPDAEC3EMLZ7DUHJAVCNFSM6AAAAABRG4TCW2VHI2DSMVQWIX3LMV43ASLTON2WKOZSGYZTKOBXHE4DIMA> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

[Timmmm]

To be clear when you say ops are reserved, it means they may still be implemented. If so that matches my understanding. The writability of the bit doesn't affect the meaning so I think you can simplify your table to:

misa.M	Zmmul	Multiply	Division
1	Implied	Legal	Legal
0	Yes	Legal	Reserved (may or may not be implemented)
0	No	Reserved (may or may not be implemented)	Reserved (may or may not be implemented)

Does that look right?

[allenjbaum]

That works for me; specifically, it doesn't depend on whether misa.M is RW or not, and the rest matches how reserved is defined.

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On Wed, Nov 6, 2024 at 4:40 AM Tim Hutt ***@***.***> wrote: To be clear when you say ops are reserved, it means they *may* still be implemented. If so that matches my understanding. The writability of the bit doesn't affect the meaning so I think you can simplify your table to: misa.M Zmmul Multiply Division 1 Implied Legal Legal 0 Yes Legal Reserved (*may or may not* be implemented) 0 No Reserved (*may or may not* be implemented) Reserved (*may or may not* be implemented) Does that look right? — Reply to this email directly, view it on GitHub <#1711 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AHPXVJUHFGZOSWKLLQ5FWC3Z7IE3TAVCNFSM6AAAAABRG4TCW2VHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDINJZGY2DSOJRGI> . You are receiving this because you commented.Message ID: ***@***.***>