ref: 8188b4f4f0e07b6669e6ae3c6c1099af917eaab4
dir: /sys/src/9/teg2/notes/bug.rfe/
/*
* return from user-mode exception.
* expects new SPSR in R0. R13 must point to ureg->type.
*/
_rfue:
TEXT rfue(SB), 1, $-4
// CPSID
// BIC $PsrMbz, R0 /* force little-endian upon return */
MOVW R0, SPSR /* ... */
/*
* order on stack is type, psr, pc, but RFEV7 needs pc, psr.
* step on type and previous word to hold temporary values.
* we could instead change the order in which psr & pc are pushed.
*/
MOVW 4(R13), R1 /* psr */
MOVW 8(R13), R2 /* pc */
MOVW R2, 4(R13) /* pc */
MOVW R1, 8(R13) /* psr */
MOVM.DB.S (R13), [R0-R14] /* restore user registers */
ADD $4, R13 /* pop type, sp -> pc */
#ifdef OLDWAY
ADD $(2*4), R13 /* pop past ureg->{type+psr} to pc */
/*
* this used to work on arm arch v[567] and still works on cpu 0.
* for some reason it sometimes sets PsrBigend on cpu 1.
* Ureg's tail was:
*
* typedef struct Ureg {
* ⋯
* ulong type; /* of exception */
* ulong psr;
* ulong pc; /* interrupted addr */
* } Ureg;
*/
RFE /* MOVM.IA.S.W (R13), [R15] */
#endif
// SETEND(0)
RFEV7W(13)