SSAMOSWAP in chapter 2.2

[jklockars]

Currently there is no mention of SSAMOSWAP in chapter 2.2.
Shouldn't there be text saying that it is an illegal instruction when the various xSSE are 0?

[ved-rivos]

The rules for SSAMOSWAP are specified in listing 4 and 5.

if privilege_mode != M && menvcfg.SSE == 0
    raise illegal-instruction exception
if S-mode not implemented
    raise illegal-instruction exception
else if privilege_mode == U && senvcfg.SSE == 0
    raise illegal-instruction exception
else if privilege_mode == VS && henvcfg.SSE == 0
    raise virtual instruction exception
else if privilege_mode == VU && senvcfg.SSE == 0
    raise virtual instruction exception
else
   <<swap>>

The SSAMOSWAP, unlike other shadow stack instructions are not Zimop. Further, while xSSE is always 0 in machine mode, the machine mode may use SSAMOSWAP to operate on memory of lower privileges using MPRV and MPP to provide the effective privilege mode for the memory access.

[ved-rivos]

Perhaps I misunderstood your comment. Were you suggesting to add statements in section 2.2 explaining the faults in listing 4/5?

[jklockars]

I am aware of the rules in chapter 2.7 (implemented this last week ;-).
Obviously, that is enough to describe the behaviour.

But for example SSRDP is described in 2.6 as doing

if (xSSE == 1)
  X(dst) = ssp
else
  X(dst) = 0
endif

and 2.2 also says what happens when xSSE = 0.
So why not similar treatment for SSAMOSWAP in 2.2?

Sure, it is not needed - I should have been clear about that.

But perhaps something like, for menvcfg:
When SSE field is 0...
...
o SSAMOSWAP raises an illegal instruction exception.

senvcfg:
...
o SSAMOSWAP raises an illegal instruction exception in U-mode, and a virtual instruction exception in VU-mode

henvcfg:
...
o SSAMOSWAP raises a virtual instruction exception.

[ved-rivos]

I will make an update to either include the listing 4 rules in 2.2 or to provide a forward reference to listing 4.

[ved-rivos]

Please check PR #193

[jklockars]

Looks good to me.

Could be "simplified" for senvcfg since there will always be an illegal instruction exception in U mode (independent of menvcfg.SSE), but that might not make anything clearer.