xtensa: new fast_alloca handler

Instead of emulating movsp instruction in the kernel use window
underflow handler to load missing register window and retry failed
movsp.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>

1 files changed, 40 insertions, 152 deletions

diff --git a/arch/xtensa/kernel/entry.S b/arch/xtensa/kernel/entry.S
index ab025c1f6e23..de1dfa18d0a1 100644
--- a/arch/xtensa/kernel/entry.S
+++ b/arch/xtensa/kernel/entry.S
@@ -31,7 +31,6 @@
 /* Unimplemented features. */
 
 #undef KERNEL_STACK_OVERFLOW_CHECK
-#undef ALLOCA_EXCEPTION_IN_IRAM
 
 /* Not well tested.
  *
@@ -819,11 +818,27 @@ ENDPROC(unrecoverable_exception)
  *
  *  The ALLOCA handler is entered when user code executes the MOVSP
  *  instruction and the caller's frame is not in the register file.
- *  In this case, the caller frame's a0..a3 are on the stack just
- *  below sp (a1), and this handler moves them.
  *
- *  For "MOVSP <ar>,<as>" without destination register a1, this routine
- *  simply moves the value from <as> to <ar> without moving the save area.
+ * This algorithm was taken from the Ross Morley's RTOS Porting Layer:
+ *
+ *    /home/ross/rtos/porting/XtensaRTOS-PortingLayer-20090507/xtensa_vectors.S
+ *
+ * It leverages the existing window spill/fill routines and their support for
+ * double exceptions. The 'movsp' instruction will only cause an exception if
+ * the next window needs to be loaded. In fact this ALLOCA exception may be
+ * replaced at some point by changing the hardware to do a underflow exception
+ * of the proper size instead.
+ *
+ * This algorithm simply backs out the register changes started by the user
+ * excpetion handler, makes it appear that we have started a window underflow
+ * by rotating the window back and then setting the old window base (OWB) in
+ * the 'ps' register with the rolled back window base. The 'movsp' instruction
+ * will be re-executed and this time since the next window frames is in the
+ * active AR registers it won't cause an exception.
+ *
+ * If the WindowUnderflow code gets a TLB miss the page will get mapped
+ * the the partial windeowUnderflow will be handeled in the double exception
+ * handler.
  *
  * Entry condition:
  *
@@ -838,155 +853,28 @@ ENDPROC(unrecoverable_exception)
  *           <  VALID_DOUBLE_EXCEPTION_ADDRESS: regular exception
  */
 
-#if XCHAL_HAVE_BE
-#define _EXTUI_MOVSP_SRC(ar)    extui ar, ar, 4, 4
-#define _EXTUI_MOVSP_DST(ar)    extui ar, ar, 0, 4
-#else
-#define _EXTUI_MOVSP_SRC(ar)    extui ar, ar, 0, 4
-#define _EXTUI_MOVSP_DST(ar)    extui ar, ar, 4, 4
-#endif
-
 ENTRY(fast_alloca)
+        rsr     a0, windowbase
+        rotw    -1
+        rsr     a2, ps
+        extui   a3, a2, PS_OWB_SHIFT, PS_OWB_WIDTH
+        xor     a3, a3, a4
+        l32i    a4, a6, PT_AREG0
+        l32i    a1, a6, PT_DEPC
+        rsr     a6, depc
+        wsr     a1, depc
+        slli    a3, a3, PS_OWB_SHIFT
+        xor     a2, a2, a3
+        wsr     a2, ps
+        rsync
 
-        /* We shouldn't be in a double exception. */
-
-        l32i    a0, a2, PT_DEPC
-        _bgeui  a0, VALID_DOUBLE_EXCEPTION_ADDRESS, .Lunhandled_double
-
-        rsr     a0, depc                # get a2
-        s32i    a4, a2, PT_AREG4        # save a4 and
-        s32i    a3, a2, PT_AREG3
-        s32i    a0, a2, PT_AREG2        # a2 to stack
-
-        /* Exit critical section. */
-
-        movi    a0, 0
-        rsr     a3, excsave1
-        s32i    a0, a3, EXC_TABLE_FIXUP
-
-        rsr     a4, epc1                # get exception address
-
-#ifdef ALLOCA_EXCEPTION_IN_IRAM
-#error  iram not supported
-#else
-        /* Note: l8ui not allowed in IRAM/IROM!! */
-        l8ui    a0, a4, 1               # read as(src) from MOVSP instruction
-#endif
-        movi    a3, .Lmovsp_src
-        _EXTUI_MOVSP_SRC(a0)            # extract source register number
-        addx8   a3, a0, a3
-        jx      a3
-
-.Lunhandled_double:
-        wsr     a0, excsave1
-        movi    a0, unrecoverable_exception
-        callx0  a0
-
-        .align 8
-.Lmovsp_src:
-        l32i    a3, a2, PT_AREG0;       _j 1f;  .align 8
-        mov     a3, a1;                 _j 1f;  .align 8
-        l32i    a3, a2, PT_AREG2;       _j 1f;  .align 8
-        l32i    a3, a2, PT_AREG3;       _j 1f;  .align 8
-        l32i    a3, a2, PT_AREG4;       _j 1f;  .align 8
-        mov     a3, a5;                 _j 1f;  .align 8
-        mov     a3, a6;                 _j 1f;  .align 8
-        mov     a3, a7;                 _j 1f;  .align 8
-        mov     a3, a8;                 _j 1f;  .align 8
-        mov     a3, a9;                 _j 1f;  .align 8
-        mov     a3, a10;                _j 1f;  .align 8
-        mov     a3, a11;                _j 1f;  .align 8
-        mov     a3, a12;                _j 1f;  .align 8
-        mov     a3, a13;                _j 1f;  .align 8
-        mov     a3, a14;                _j 1f;  .align 8
-        mov     a3, a15;                _j 1f;  .align 8
-
-1:
-
-#ifdef ALLOCA_EXCEPTION_IN_IRAM
-#error  iram not supported
-#else
-        l8ui    a0, a4, 0               # read ar(dst) from MOVSP instruction
-#endif
-        addi    a4, a4, 3               # step over movsp
-        _EXTUI_MOVSP_DST(a0)            # extract destination register
-        wsr     a4, epc1                # save new epc_1
-
-        _bnei   a0, 1, 1f               # no 'movsp a1, ax': jump
-
-        /* Move the save area. This implies the use of the L32E
-         * and S32E instructions, because this move must be done with
-         * the user's PS.RING privilege levels, not with ring 0
-         * (kernel's) privileges currently active with PS.EXCM
-         * set. Note that we have stil registered a fixup routine with the
-         * double exception vector in case a double exception occurs.
-         */
-
-        /* a0,a4:avail a1:old user stack a2:exc. stack a3:new user stack. */
-
-        l32e    a0, a1, -16
-        l32e    a4, a1, -12
-        s32e    a0, a3, -16
-        s32e    a4, a3, -12
-        l32e    a0, a1, -8
-        l32e    a4, a1, -4
-        s32e    a0, a3, -8
-        s32e    a4, a3, -4
-
-        /* Restore stack-pointer and all the other saved registers. */
-
-        mov     a1, a3
-
-        l32i    a4, a2, PT_AREG4
-        l32i    a3, a2, PT_AREG3
-        l32i    a0, a2, PT_AREG0
-        l32i    a2, a2, PT_AREG2
-        rfe
-
-        /*  MOVSP <at>,<as>  was invoked with <at> != a1.
-         *  Because the stack pointer is not being modified,
-         *  we should be able to just modify the pointer
-         *  without moving any save area.
-         *  The processor only traps these occurrences if the
-         *  caller window isn't live, so unfortunately we can't
-         *  use this as an alternate trap mechanism.
-         *  So we just do the move.  This requires that we
-         *  resolve the destination register, not just the source,
-         *  so there's some extra work.
-         *  (PERHAPS NOT REALLY NEEDED, BUT CLEANER...)
-         */
-
-        /* a0 dst-reg, a1 user-stack, a2 stack, a3 value of src reg. */
-
-1:      movi    a4, .Lmovsp_dst
-        addx8   a4, a0, a4
-        jx      a4
-
-        .align 8
-.Lmovsp_dst:
-        s32i    a3, a2, PT_AREG0;       _j 1f;  .align 8
-        mov     a1, a3;                 _j 1f;  .align 8
-        s32i    a3, a2, PT_AREG2;       _j 1f;  .align 8
-        s32i    a3, a2, PT_AREG3;       _j 1f;  .align 8
-        s32i    a3, a2, PT_AREG4;       _j 1f;  .align 8
-        mov     a5, a3;                 _j 1f;  .align 8
-        mov     a6, a3;                 _j 1f;  .align 8
-        mov     a7, a3;                 _j 1f;  .align 8
-        mov     a8, a3;                 _j 1f;  .align 8
-        mov     a9, a3;                 _j 1f;  .align 8
-        mov     a10, a3;                _j 1f;  .align 8
-        mov     a11, a3;                _j 1f;  .align 8
-        mov     a12, a3;                _j 1f;  .align 8
-        mov     a13, a3;                _j 1f;  .align 8
-        mov     a14, a3;                _j 1f;  .align 8
-        mov     a15, a3;                _j 1f;  .align 8
-
-1:      l32i    a4, a2, PT_AREG4
-        l32i    a3, a2, PT_AREG3
-        l32i    a0, a2, PT_AREG0
-        l32i    a2, a2, PT_AREG2
-        rfe
-
+        _bbci.l a4, 31, 4f
+        rotw    -1
+        _bbci.l a8, 30, 8f
+        rotw    -1
+        j       _WindowUnderflow12
+8:      j       _WindowUnderflow8
+4:      j       _WindowUnderflow4
 ENDPROC(fast_alloca)
 
 /*