powerpc: Endian safe trampoline

Create a trampoline that works in either endian and flips to
the expected endian. Use it for primary and secondary thread
entry as well as RTAS and OF call return.

Credit for finding the magic instruction goes to Paul Mackerras

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

3 files changed, 53 insertions, 17 deletions

diff --git a/arch/powerpc/include/asm/ppc_asm.h b/arch/powerpc/include/asm/ppc_asm.h
index 0c51fb4fd2aa..ce05bba0bfc6 100644
--- a/arch/powerpc/include/asm/ppc_asm.h
+++ b/arch/powerpc/include/asm/ppc_asm.h
@@ -845,6 +845,35 @@ END_FTR_SECTION_NESTED(CPU_FTR_HAS_PPR,CPU_FTR_HAS_PPR,946)
 #define N_SLINE 68
 #define N_SO    100
 
-#endif /*  __ASSEMBLY__ */
+/*
+ * Create an endian fixup trampoline
+ *
+ * This starts with a "tdi 0,0,0x48" instruction which is
+ * essentially a "trap never", and thus akin to a nop.
+ *
+ * The opcode for this instruction read with the wrong endian
+ * however results in a b . + 8
+ *
+ * So essentially we use that trick to execute the following
+ * trampoline in "reverse endian" if we are running with the
+ * MSR_LE bit set the "wrong" way for whatever endianness the
+ * kernel is built for.
+ */
 
+#ifdef CONFIG_PPC_BOOK3E
+#define FIXUP_ENDIAN
+#else
+#define FIXUP_ENDIAN                                               \
+        tdi   0,0,0x48;   /* Reverse endian of b . + 8          */ \
+        b     $+36;       /* Skip trampoline if endian is good  */ \
+        .long 0x05009f42; /* bcl 20,31,$+4                      */ \
+        .long 0xa602487d; /* mflr r10                           */ \
+        .long 0x1c004a39; /* addi r10,r10,28                    */ \
+        .long 0xa600607d; /* mfmsr r11                          */ \
+        .long 0x01006b69; /* xori r11,r11,1                     */ \
+        .long 0xa6035a7d; /* mtsrr0 r10                         */ \
+        .long 0xa6037b7d; /* mtsrr1 r11                         */ \
+        .long 0x2400004c  /* rfid                               */
+#endif /* !CONFIG_PPC_BOOK3E */
+#endif /*  __ASSEMBLY__ */
 #endif /* _ASM_POWERPC_PPC_ASM_H */
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index c04cdf70d487..889ea2b19383 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -1017,7 +1017,7 @@ _GLOBAL(enter_rtas)
         
         li      r9,1
         rldicr  r9,r9,MSR_SF_LG,(63-MSR_SF_LG)
-        ori     r9,r9,MSR_IR|MSR_DR|MSR_FE0|MSR_FE1|MSR_FP|MSR_RI
+        ori     r9,r9,MSR_IR|MSR_DR|MSR_FE0|MSR_FE1|MSR_FP|MSR_RI|MSR_LE
         andc    r6,r0,r9
         sync                            /* disable interrupts so SRR0/1 */
         mtmsrd  r0                      /* don't get trashed */
@@ -1032,6 +1032,8 @@ _GLOBAL(enter_rtas)
         b       .       /* prevent speculative execution */
 
 _STATIC(rtas_return_loc)
+        FIXUP_ENDIAN
+
         /* relocation is off at this point */
         GET_PACA(r4)
         clrldi  r4,r4,2                 /* convert to realmode address */
@@ -1103,28 +1105,30 @@ _GLOBAL(enter_prom)
         std     r10,_CCR(r1)
         std     r11,_MSR(r1)
 
-        /* Get the PROM entrypoint */
-        mtlr    r4
+        /* Put PROM address in SRR0 */
+        mtsrr0  r4
+
+        /* Setup our trampoline return addr in LR */
+        bcl     20,31,$+4
+0:      mflr    r4
+        addi    r4,r4,(1f - 0b)
+        mtlr    r4
 
-        /* Switch MSR to 32 bits mode
+        /* Prepare a 32-bit mode big endian MSR
          */
 #ifdef CONFIG_PPC_BOOK3E
         rlwinm  r11,r11,0,1,31
-        mtmsr   r11
+        mtsrr1  r11
+        rfi
 #else /* CONFIG_PPC_BOOK3E */
-        mfmsr   r11
-        li      r12,1
-        rldicr  r12,r12,MSR_SF_LG,(63-MSR_SF_LG)
-        andc    r11,r11,r12
-        li      r12,1
-        rldicr  r12,r12,MSR_ISF_LG,(63-MSR_ISF_LG)
-        andc    r11,r11,r12
-        mtmsrd  r11
+        LOAD_REG_IMMEDIATE(r12, MSR_SF | MSR_ISF | MSR_LE)
+        andc    r11,r11,r12
+        mtsrr1  r11
+        rfid
 #endif /* CONFIG_PPC_BOOK3E */
-        isync
 
-        /* Enter PROM here... */
-        blrl
+1:      /* Return from OF */
+        FIXUP_ENDIAN
 
         /* Just make sure that r1 top 32 bits didn't get
          * corrupt by OF
diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
index 3d11d8038dee..2ae41aba4053 100644
--- a/arch/powerpc/kernel/head_64.S
+++ b/arch/powerpc/kernel/head_64.S
@@ -68,6 +68,7 @@ _stext:
 _GLOBAL(__start)
         /* NOP this out unconditionally */
 BEGIN_FTR_SECTION
+        FIXUP_ENDIAN
         b       .__start_initialization_multiplatform
 END_FTR_SECTION(0, 1)
 
@@ -115,6 +116,7 @@ __run_at_load:
  */
         .globl  __secondary_hold
 __secondary_hold:
+        FIXUP_ENDIAN
 #ifndef CONFIG_PPC_BOOK3E
         mfmsr   r24
         ori     r24,r24,MSR_RI
@@ -205,6 +207,7 @@ _GLOBAL(generic_secondary_thread_init)
  * as SCOM before entry).
  */
 _GLOBAL(generic_secondary_smp_init)
+        FIXUP_ENDIAN
         mr      r24,r3
         mr      r25,r4