Clarify LPAD exceptions

[henry-hsieh]

The spec said:

When Zicfilp is enabled, LPAD is the only instruction allowed to execute when the ELP state is LP_EXPECTED.

It's expected that non-LPAD instructions will also raise the software-check exceptions. I added the descriptions in both the listings and the operations.

[ved-rivos]

This change is not required and is not accurate. The property that the next instruction in the instruction stream must be LPAD is a decode time property. Whereas the listing modified by this PR is for the LPAD instruction. So the check else if (inst[6:0] != 23 || inst[11:7] != 0) is always false since this instruction has already been decoded as LPAD.

The rule that the next instruction in the stream must be LPAD has been established here:

When ELP is set to LP_EXPECTED, if the next instruction in the instruction stream is not 4-byte aligned, or is not LPAD, or if the landing pad label encoded in LPAD is not zero and does not match the expected landing pad label in bits 31:12 of the x7 register, then a software-check exception (cause=18) with xtval set to "landing pad fault (code=2)" is raised else the ELP is updated to NO_LP_EXPECTED.

The "is not LPAD" is a decode check. Once it has been decoded as a LPAD, the operations in listing 7 apply.

[henry-hsieh]

Thanks! I misunderstood the checks. I'll close the PR.