/* * arch/xtensa/kernel/vectors.S * * This file contains all exception vectors (user, kernel, and double), * as well as the window vectors (overflow and underflow), and the debug * vector. These are the primary vectors executed by the processor if an * exception occurs. * * This file is subject to the terms and conditions of the GNU General * Public License. See the file "COPYING" in the main directory of * this archive for more details. * * Copyright (C) 2005 - 2008 Tensilica, Inc. * * Chris Zankel <chris@zankel.net> *
*/
/* * We use a two-level table approach. The user and kernel exception vectors * use a first-level dispatch table to dispatch the exception to a registered * fast handler or the default handler, if no fast handler was registered. * The default handler sets up a C-stack and dispatches the exception to a * registerd C handler in the second-level dispatch table. * * Fast handler entry condition: * * a0: trashed, original value saved on stack (PT_AREG0) * a1: a1 * a2: new stack pointer, original value in depc * a3: dispatch table * depc: a2, original value saved on stack (PT_DEPC) * excsave_1: a3 * * The value for PT_DEPC saved to stack also functions as a boolean to * indicate that the exception is either a double or a regular exception: * * PT_DEPC >= VALID_DOUBLE_EXCEPTION_ADDRESS: double exception * < VALID_DOUBLE_EXCEPTION_ADDRESS: regular exception * * Note: Neither the kernel nor the user exception handler generate literals. *
*/
/* * User exception vector. (Exceptions with PS.UM == 1, PS.EXCM == 0) * * We get here when an exception occurred while we were in userland. * We switch to the kernel stack and jump to the first level handler * associated to the exception cause. * * Note: the saved kernel stack pointer (EXC_TABLE_KSTK) is already * decremented by PT_USER_SIZE.
*/
.section .UserExceptionVector.text, "ax"
ENTRY(_UserExceptionVector)
xsr a3, excsave1 # save a3 and get dispatch table
wsr a2, depc # save a2
l32i a2, a3, EXC_TABLE_KSTK # load kernel stack to a2
s32i a0, a2, PT_AREG0 # save a0 to ESF
rsr a0, exccause # retrieve exception cause
s32i a0, a2, PT_DEPC # mark it as a regular exception
addx4 a0, a0, a3 # find entry in table
l32i a0, a0, EXC_TABLE_FAST_USER # load handler
xsr a3, excsave1 # restore a3 and dispatch table
jx a0
ENDPROC(_UserExceptionVector)
/* * Kernel exception vector. (Exceptions with PS.UM == 0, PS.EXCM == 0) * * We get this exception when we were already in kernel space. * We decrement the current stack pointer (kernel) by PT_KERNEL_SIZE and * jump to the first-level handler associated with the exception cause. * * Note: we need to preserve space for the spill region.
*/
.section .KernelExceptionVector.text, "ax"
ENTRY(_KernelExceptionVector)
xsr a3, excsave1 # save a3, and get dispatch table
wsr a2, depc # save a2
addi a2, a1, -16 - PT_KERNEL_SIZE # adjust stack pointer
s32i a0, a2, PT_AREG0 # save a0 to ESF
rsr a0, exccause # retrieve exception cause
s32i a0, a2, PT_DEPC # mark it as a regular exception
addx4 a0, a0, a3 # find entry in table
l32i a0, a0, EXC_TABLE_FAST_KERNEL # load handler address
xsr a3, excsave1 # restore a3 and dispatch table
jx a0
ENDPROC(_KernelExceptionVector)
/* * Double exception vector (Exceptions with PS.EXCM == 1) * We get this exception when another exception occurs while were are * already in an exception, such as window overflow/underflow exception, * or 'expected' exceptions, for example memory exception when we were trying * to read data from an invalid address in user space. * * Note that this vector is never invoked for level-1 interrupts, because such * interrupts are disabled (masked) when PS.EXCM is set. * * We decode the exception and take the appropriate action. However, the * double exception vector is much more careful, because a lot more error * cases go through the double exception vector than through the user and * kernel exception vectors. * * Occasionally, the kernel expects a double exception to occur. This usually * happens when accessing user-space memory with the user's permissions * (l32e/s32e instructions). The kernel state, though, is not always suitable * for immediate transfer of control to handle_double, where "normal" exception * processing occurs. Also in kernel mode, TLB misses can occur if accessing * vmalloc memory, possibly requiring repair in a double exception handler. * * The variable at TABLE_FIXUP offset from the pointer in EXCSAVE_1 doubles as * a boolean variable and a pointer to a fixup routine. If the variable * EXC_TABLE_FIXUP is non-zero, this handler jumps to that address. A value of * zero indicates to use the default kernel/user exception handler. * There is only one exception, when the value is identical to the exc_table * label, the kernel is in trouble. This mechanism is used to protect critical * sections, mainly when the handler writes to the stack to assert the stack * pointer is valid. Once the fixup/default handler leaves that area, the * EXC_TABLE_FIXUP variable is reset to the fixup handler or zero. * * Procedures wishing to use this mechanism should set EXC_TABLE_FIXUP to the * nonzero address of a fixup routine before it could cause a double exception * and reset it before it returns. * * Some other things to take care of when a fast exception handler doesn't * specify a particular fixup handler but wants to use the default handlers: * * - The original stack pointer (in a1) must not be modified. The fast * exception handler should only use a2 as the stack pointer. * * - If the fast handler manipulates the stack pointer (in a2), it has to * register a valid fixup handler and cannot use the default handlers. * * - The handler can use any other generic register from a3 to a15, but it * must save the content of these registers to stack (PT_AREG3...PT_AREGx) * * - These registers must be saved before a double exception can occur. * * - If we ever implement handling signals while in double exceptions, the * number of registers a fast handler has saved (excluding a0 and a1) must * be written to PT_AREG1. (1 if only a3 is used, 2 for a3 and a4, etc. ) * * The fixup handlers are special handlers: * * - Fixup entry conditions differ from regular exceptions: * * a0: DEPC * a1: a1 * a2: trashed, original value in EXC_TABLE_DOUBLE_SAVE * a3: exctable * depc: a0 * excsave_1: a3 * * - When the kernel enters the fixup handler, it still assumes it is in a * critical section, so EXC_TABLE_FIXUP variable is set to exc_table. * The fixup handler, therefore, has to re-register itself as the fixup * handler before it returns from the double exception. * * - Fixup handler can share the same exception frame with the fast handler. * The kernel stack pointer is not changed when entering the fixup handler. * * - Fixup handlers can jump to the default kernel and user exception * handlers. Before it jumps, though, it has to setup a exception frame * on stack. Because the default handler resets the register fixup handler * the fixup handler must make sure that the default handler returns to * it instead of the exception address, so it can re-register itself as * the fixup handler. * * In case of a critical condition where the kernel cannot recover, we jump * to unrecoverable_exception with the following entry conditions. * All registers a0...a15 are unchanged from the last exception, except: * * a0: last address before we jumped to the unrecoverable_exception. * excsave_1: a0 * * * See the handle_alloca_user and spill_registers routines for example clients. * * FIXME: Note: we currently don't allow signal handling coming from a double * exception, so the item markt with (*) is not required.
*/
/* Check for kernel double exception (usually fatal). */
rsr a2, ps
_bbsi.l a2, PS_UM_BIT, 1f
j .Lksp
.align 4
.literal_position
1: /* Check if we are currently handling a window exception. */ /* Note: We don't need to indicate that we enter a critical section. */
/* * Restart window underflow exception. * Currently: * depc = orig a0, * a0 = orig DEPC, * a2 = new sp based on KSTK from exc_table * a3 = excsave_1 * excsave_1 = orig a3 * * We return to the instruction in user space that caused the window * underflow exception. Therefore, we change window base to the value * before we entered the window underflow exception and prepare the * registers to return as if we were coming from a regular exception * by changing depc (in a0). * Note: We can trash the current window frame (a0...a3) and depc!
*/
_DoubleExceptionVector_WindowUnderflow:
xsr a3, excsave1
wsr a2, depc # save stack pointer temporarily
rsr a0, ps
extui a0, a0, PS_OWB_SHIFT, PS_OWB_WIDTH
wsr a0, windowbase
rsync
/* We are now in the previous window frame. Save registers again. */
xsr a2, depc # save a2 and get stack pointer
s32i a0, a2, PT_AREG0
xsr a3, excsave1
rsr a0, exccause
s32i a0, a2, PT_DEPC # mark it as a regular exception
addx4 a0, a0, a3
xsr a3, excsave1
l32i a0, a0, EXC_TABLE_FAST_USER
jx a0
#else
j .Lfixup
#endif
/* * We only allow the ITLB miss exception if we are in kernel space. * All other exceptions are unexpected and thus unrecoverable!
*/
#ifdef SUPPORT_WINDOWED /* * Restart window OVERFLOW exception. * Currently: * depc = orig a0, * a0 = orig DEPC, * a2 = new sp based on KSTK from exc_table * a3 = EXCSAVE_1 * excsave_1 = orig a3 * * We return to the instruction in user space that caused the window * overflow exception. Therefore, we change window base to the value * before we entered the window overflow exception and prepare the * registers to return as if we were coming from a regular exception * by changing DEPC (in a0). * * NOTE: We CANNOT trash the current window frame (a0...a3), but we * can clobber depc. * * The tricky part here is that overflow8 and overflow12 handlers * save a0, then clobber a0. To restart the handler, we have to restore * a0 if the double exception was past the point where a0 was clobbered. * * To keep things simple, we take advantage of the fact all overflow * handlers save a0 in their very first instruction. If DEPC was past * that instruction, we can safely restore a0 from where it was saved * on the stack. * * a0: depc, a1: a1, a2: kstk, a3: exc_table, depc: a0, excsave1: a3
*/
_DoubleExceptionVector_WindowOverflow:
extui a2, a0, 0, 6 # get offset into 64-byte vector handler
beqz a2, 1f # if at start of vector, don't restore
addi a0, a0, -128
bbsi.l a0, 8, 1f # don't restore except for overflow 8 and 12
/* * This fixup handler is for the extremely unlikely case where the * overflow handler's reference thru a0 gets a hardware TLB refill * that bumps out the (distinct, aliasing) TLB entry that mapped its * prior references thru a9/a13, and where our reference now thru * a9/a13 gets a 2nd-level miss exception (not hardware TLB refill).
*/
movi a2, window_overflow_restore_a0_fixup
s32i a2, a3, EXC_TABLE_FIXUP
l32i a2, a3, EXC_TABLE_DOUBLE_SAVE
xsr a3, excsave1
bbsi.l a0, 7, 2f
/* * Restore a0 as saved by _WindowOverflow8().
*/
2: /* * Restore a0 as saved by _WindowOverflow12().
*/
l32e a0, a13, -16
wsr a0, depc # replace the saved a0
3:
xsr a3, excsave1
movi a0, 0
s32i a0, a3, EXC_TABLE_FIXUP
s32i a2, a3, EXC_TABLE_DOUBLE_SAVE
1: /* * Restore WindowBase while leaving all address registers restored. * We have to use ROTW for this, because WSR.WINDOWBASE requires * an address register (which would prevent restore). * * Window Base goes from 0 ... 7 (Module 8) * Window Start is 8 bits; Ex: (0b1010 1010):0x55 from series of call4s
*/
/* * We are now in the user code's original window frame. * Process the exception as a user exception as if it was * taken by the user code. * * This is similar to the user exception vector, * except that PT_DEPC isn't set to EXCCAUSE.
*/
1:
xsr a3, excsave1
wsr a2, depc
l32i a2, a3, EXC_TABLE_KSTK
s32i a0, a2, PT_AREG0
rsr a0, exccause
/* * Fixup handler for TLB miss in double exception handler for window owerflow. * We get here with windowbase set to the window that was being spilled and * a0 trashed. a0 bit 7 determines if this is a call8 (bit clear) or call12 * (bit set) window. * * We do the following here: * - go to the original window retaining a0 value; * - set up exception stack to return back to appropriate a0 restore code * (we'll need to rotate window back and there's no place to save this * information, use different return address for that); * - handle the exception; * - go to the window that was being spilled; * - set up window_overflow_restore_a0_fixup as a fixup routine; * - reload a0; * - restore the original window; * - reset the default fixup routine; * - return to user. By the time we get to this fixup handler all information * about the conditions of the original double exception that happened in * the window overflow handler is lost, so we just return to userspace to * retry overflow from start. * * a0: value of depc, original value in depc * a2: trashed, original value in EXC_TABLE_DOUBLE_SAVE * a3: exctable, original value in excsave1
*/
rotw \n /* Need to preserve a0 value here to be able to handle exception * that may occur on a0 reload from stack. It may occur because * TLB miss handler may not be atomic and pointer to page table * may be lost before we get here. There are no free registers, * so we need to use EXC_TABLE_DOUBLE_SAVE area.
*/
xsr a3, excsave1
s32i a2, a3, EXC_TABLE_DOUBLE_SAVE
movi a2, window_overflow_restore_a0_fixup
s32i a2, a3, EXC_TABLE_FIXUP
l32i a2, a3, EXC_TABLE_DOUBLE_SAVE
xsr a3, excsave1
bbsi.l a0, 7, 1f
l32e a0, a9, -16
j 2f
1:
l32e a0, a13, -16
2:
rotw -\n
/* * Debug interrupt vector * * There is not much space here, so simply jump to another handler. * EXCSAVE[DEBUGLEVEL] has been set to that handler.
*/
/* * Medium priority level interrupt vectors * * Each takes less than 16 (0x10) bytes, no literals, by placing * the extra 8 bytes that would otherwise be required in the window * vectors area where there is space. With relocatable vectors, * all vectors are within ~ 4 kB range of each other, so we can * simply jump (J) to another vector without having to use JX. * * common_exception code gets current IRQ level in PS.INTLEVEL * and preserves it for the IRQ handling time.
*/
/* Window overflow and underflow handlers. * The handlers must be 64 bytes apart, first starting with the underflow * handlers underflow-4 to underflow-12, then the overflow handlers * overflow-4 to overflow-12. * * Note: We rerun the underflow handlers if we hit an exception, so * we try to access any page that would cause a page fault early.
*/
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