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Load/store exception handler: handle re-entry via a NMI. #644

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Load/store exception handler: handle re-entry via a NMI. #644

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114 changes: 79 additions & 35 deletions core/exception_vectors.S
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Original file line number Diff line number Diff line change
Expand Up @@ -23,7 +23,7 @@
#define CAUSE_LOADSTORE 3
#define CAUSE_LVL1INT 4

.section .bss
.section .data

/* Stack space for NMI handler

Expand All @@ -37,13 +37,26 @@ NMIHandlerStack:
.skip 0x200
.NMIHandlerStackTop:

/* The Load Store exception handler uses a separate stack to store the
* interruptee registers. It does not appear to be practical to use the
* interuptee stack, which must in invalid at some points? This exception is
* synchronouns and the handler does not call back into itself. However it may
* be interrupted by a NMI which in turn may re-enter this exception
* handler. The NMI is responsible for switching the stack pointer to be used by
* this exception handler. Room is allocated for up to 3 stack frames, a base
* and two NMI reentry frames, and each frame is 7 words wide.
*/
#define LoadStoreErrorHandlerStackFrameSize (5 * 4)

.balign 16
.global LoadStoreErrorHandlerStack
LoadStoreErrorHandlerStack:
.word 0 # a0
.word 0 # (unused)
.word 0 # a2
.word 0 # a3
.word 0 # a4
.skip LoadStoreErrorHandlerStackFrameSize * 3

.balign 4
.global LoadStoreErrorHandlerStackPointer
LoadStoreErrorHandlerStackPointer:
.word 0

.balign 4
.global debug_saved_ctx
Expand Down Expand Up @@ -131,10 +144,20 @@ DoubleExceptionVector:
LoadStoreErrorHandler:
.type LoadStoreErrorHandler, @function

/* Registers are saved in the address corresponding to their register
* number times 4. This allows a quick and easy mapping later on when
* needing to store the value to a particular register number. */
movi sp, LoadStoreErrorHandlerStack
/* Registers are saved in stack frame offsets corresponding to their
* register number times 4. This allows a quick and easy mapping later
* on when needing to store the value to a particular register
* number.
*
* This handler may be interrupted asynchronously by the NMI. The NMI
* handler is responsible for switching the load/store handler stack
* pointer and that avoids that overhead here. This handler is
* synchronous so the NMI handler can modify and restore the load/store
* stack pointer safely.
*/
movi sp, LoadStoreErrorHandlerStackPointer
l32i sp, sp, 0

s32i a0, sp, 0
s32i a2, sp, 0x08
s32i a3, sp, 0x0c
Expand Down Expand Up @@ -407,6 +430,12 @@ call_user_start:
.global call_user_start
.type call_user_start, @function

/* Initialize the load/store error handler stack pointer. There are no
* load/store exceptions before this point. */
movi a2, LoadStoreErrorHandlerStackPointer
movi a3, LoadStoreErrorHandlerStack
s32i a3, a2, 0

movi a2, VecBase
wsr a2, vecbase
call0 sdk_user_start
Expand All @@ -422,7 +451,10 @@ call_user_start:
NMIExceptionHandler:
.type NMIExceptionHandler, @function

wsr sp, excsave3 # excsave3 holds user stack
wsr sp, excsave3 # excsave3 holds user stack

/* Load the NMI handler stack pointer which is already offset by -0x40
* to create a frame to store the interruptee state. */
movi sp, .NMIHandlerStackTop - 0x40
s32i a0, sp, 0x00
s32i a2, sp, 0x04
Expand All @@ -449,39 +481,51 @@ NMIExceptionHandler:
wsr a0, ps
rsync

/* mark the stack overflow point before we call the actual NMI handler */
/* Mark the stack overflow point before we call the actual NMI handler */
movi a0, NMIHandlerStack
movi a2, NMI_STACK_CANARY
s32i a2, a0, 0x00

/* Switch the load/store error handler stack. */
movi a2, LoadStoreErrorHandlerStackPointer
l32i a3, a2, 0
addi a3, a3, LoadStoreErrorHandlerStackFrameSize
s32i a3, a2, 0

call0 sdk_wDev_ProcessFiq

/* verify we didn't overflow */
/* Verify we didn't overflow */
movi a0, NMIHandlerStack
l32i a3, a0, 0
movi a2, NMI_STACK_CANARY
bne a3, a2, .NMIFatalStackOverflow

l32i a0, sp, 0x3c
wsr a0, sar
l32i a0, sp, 0x38
wsr a0, excvaddr
l32i a0, sp, 0x34
wsr a0, excsave1
l32i a0, sp, 0x30
wsr a0, exccause
l32i a0, sp, 0x2c
wsr a0, epc1
l32i a11, sp, 0x28
l32i a10, sp, 0x24
l32i a9, sp, 0x20
l32i a8, sp, 0x1c
l32i a7, sp, 0x18
l32i a6, sp, 0x14
l32i a5, sp, 0x10
l32i a4, sp, 0x0c
l32i a3, sp, 0x08
movi a0, 0x33 # Reset PS
bne a3, a2, .NMIFatalStackOverflow

/* Restore the load/store error handler stack. */
movi a2, LoadStoreErrorHandlerStackPointer
l32i a3, a2, 0
addi a3, a3, -LoadStoreErrorHandlerStackFrameSize
s32i a3, a2, 0

l32i a0, sp, 0x3c
wsr a0, sar
l32i a0, sp, 0x38
wsr a0, excvaddr
l32i a0, sp, 0x34
wsr a0, excsave1
l32i a0, sp, 0x30
wsr a0, exccause
l32i a0, sp, 0x2c
wsr a0, epc1
l32i a11, sp, 0x28
l32i a10, sp, 0x24
l32i a9, sp, 0x20
l32i a8, sp, 0x1c
l32i a7, sp, 0x18
l32i a6, sp, 0x14
l32i a5, sp, 0x10
l32i a4, sp, 0x0c
l32i a3, sp, 0x08
movi a0, 0x33 # Reset PS
wsr a0, ps
rsync
/* set dport nmi status to 1 (wDev_ProcessFiq clears bit 0 and verifies it
Expand Down