Possibility of 'Environment call from S-mode' delegation to S-mode

[evgeniy-paltsev]

The common sense about how ECALLs are used gives me impression that S-ECALL shouldn't be delegatable to S-mode.

It was mentioned several times in this repo issues/PRs that S-ECALL shouldn't be delegated to S-mode, i.e.: #112 (comment), #712 (comment)

In similar case of Environment call from VS-mode delegation to VS-mode the docs clearly forbids such delegation - as corresponding bits of hedeleg must be Read-only 0 according to docs.

However, the recent riscv-privileged documentation doesn't have any restrictions about S-ECALL delegation to S-mode, and scause register has 9 - Environment call from S-mode as a valid value. Moreover, the S-ECALL delegation to S-mode still works in spike built from recent sources.

So, does this mean that we allow implementations to have ECALL from S-mode delegatable to S-mode?

[aswaterman]

The spec doesn't mandate that medeleg[9] be hardwired to 0, so it is legal to build an implementation that supports this delegation. However, setting medeleg[9]=1 would effectively prevent correct implementation of the SBI, and so doing so would in most contexts be considered an M-mode software bug.