SIE in software for RISC-V?

gistfile1.txt

The first thing we need to do when invoking SIE is save the current thread state.
The invokING thread called us via an ECALL, so some of this state is in mstatus and mepc.
So, save it.

        _SIE:   ; A0 -> SIEBK to invoke.
                csrrw   t0, mscratch, x0        ; mscratch = t0, t0 = pointer to current SIEBK.
                sd      x1, _X1(t0)
                sd      x2, _X2(t0)
                ...
                sd      x31, _X31(t0)
                addi    x1, t0, 0
                sd      x1, _X5(t0)             ; or whatever it maps to; I forget off-hand.
                csrrw   x1, mstatus, x0         ; Get current mstatus
                sd      x1, _MSTATUS(t0)
                csrrw   x1, mepc, x0
                sd      x1, _MEPC(t0)
                ; save other CSRs here, such as page tables, and whatnot.
                
Now that we have our current thread state saved, we need to switch to the new SIEBK.
Because exceptions in the invokED task will cause a return to the invokING task,
we need to establish the back-link.

                sd      t0, _ReturnLink(a0)
                addi    t0, a0, 0               ; Assume the new task identity
                
                ; ... reload important CSRs here in reverse order.
                ld      x1, _MEPC(t0)
                csrrw   x0, mepc, x1
                ld      x1, _MSTATUS(t0)
                csrrw   x0, mstatus, x1         ; Pray this doesn't put us into a bizarre state.
                ld      x1, _X5(t0)
                csrrw   x0, mscratch, x1
                ld      x31, _X31(t0)
                ld      x30, _X30(t0)
                ....
                ld      x2, _X2(t0)
                ld      x1, _X1(t0)
                csrrw   t0, mscratch, t0
                mret                            ; BOOM, start running new VM thread here.