powerpc/40x: Don't use SPRN_SPRG_SCRATCH2 in EXCEPTION_PROLOG

Unlike said in the comment, r1 is not reused by the critical
exception handler, as it uses a dedicated critirq_ctx stack.
Decrementing r1 early is then unneeded.

Should the above be valid, the code is crap buggy anyway as
r1 gets some intermediate values that would jeopardise the
whole process (for instance after mfspr   r1,SPRN_SPRG_THREAD)

Using SPRN_SPRG_SCRATCH2 to save r1 is then not needed, r11 can be
used instead. This avoids one mtspr and one mfspr and makes the
prolog closer to what's done on 6xx and 8xx.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

diff --git a/arch/powerpc/kernel/head_40x.S b/arch/powerpc/kernel/head_40x.S
index efa219d2136e74a77eb0882395c305d523aadbc2..fa033203dcdbb624dbb97eb9d5f9c47642b4fb51 100644 (file)
--- a/arch/powerpc/kernel/head_40x.S
+++ b/arch/powerpc/kernel/head_40x.S
@@ -102,23 +102,20 @@ _ENTRY(saved_ksp_limit)
  * Exception vector entry code. This code runs with address translation
  * turned off (i.e. using physical addresses). We assume SPRG_THREAD has
  * the physical address of the current task thread_struct.
- * Note that we have to have decremented r1 before we write to any fields
- * of the exception frame, since a critical interrupt could occur at any
- * time, and it will write to the area immediately below the current r1.
  */
 #define NORMAL_EXCEPTION_PROLOG                                                     \
        mtspr   SPRN_SPRG_SCRATCH0,r10; /* save two registers to work with */\
        mtspr   SPRN_SPRG_SCRATCH1,r11;                                      \
-       mtspr   SPRN_SPRG_SCRATCH2,r1;                                       \
        mfcr    r10;                    /* save CR in r10 for now          */\
        mfspr   r11,SPRN_SRR1;          /* check whether user or kernel    */\
        andi.   r11,r11,MSR_PR;                                              \
-       beq     1f;                                                          \
-       mfspr   r1,SPRN_SPRG_THREAD;    /* if from user, start at top of   */\
-       lwz     r1,TASK_STACK-THREAD(r1); /* this thread's kernel stack   */\
-       addi    r1,r1,THREAD_SIZE;                                           \
-1:     subi    r1,r1,INT_FRAME_SIZE;   /* Allocate an exception frame     */\
        tophys(r11,r1);                                                      \
+       beq     1f;                                                          \
+       mfspr   r11,SPRN_SPRG_THREAD;   /* if from user, start at top of   */\
+       lwz     r11,TASK_STACK-THREAD(r11); /* this thread's kernel stack */\
+       addi    r11,r11,THREAD_SIZE;                                         \
+       tophys(r11,r11);                                                     \
+1:     subi    r11,r11,INT_FRAME_SIZE; /* Allocate an exception frame     */\
        stw     r10,_CCR(r11);          /* save various registers          */\
        stw     r12,GPR12(r11);                                              \
        stw     r9,GPR9(r11);                                                \
@@ -128,11 +125,11 @@ _ENTRY(saved_ksp_limit)
        stw     r12,GPR11(r11);                                              \
        mflr    r10;                                                         \
        stw     r10,_LINK(r11);                                              \
-       mfspr   r10,SPRN_SPRG_SCRATCH2;                                      \
        mfspr   r12,SPRN_SRR0;                                               \
-       stw     r10,GPR1(r11);                                               \
+       stw     r1,GPR1(r11);                                                \
        mfspr   r9,SPRN_SRR1;                                                \
-       stw     r10,0(r11);                                                  \
+       stw     r1,0(r11);                                                   \
+       tovirt(r1,r11);                 /* set new kernel sp */ \
        rlwinm  r9,r9,0,14,12;          /* clear MSR_WE (necessary?)       */\
        stw     r0,GPR0(r11);                                                \
        SAVE_4GPRS(3, r11);                                                  \