powerpc/64: remove support for kernel-mode syscalls

There is support for the kernel to execute the 'sc 0' instruction and
make a system call to itself. This is a relic that is unused in the
tree, therefore untested. It's also highly questionable for modules to
be doing this.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190827033010.28090-3-npiggin@gmail.com

arch/powerpc/kernel/entry_64.S

@@ -69,24 +69,20 @@ BEGIN_FTR_SECTION
 	bne	.Ltabort_syscall
 END_FTR_SECTION_IFSET(CPU_FTR_TM)
 #endif
-	andi.	r10,r12,MSR_PR
 	mr	r10,r1
-	addi	r1,r1,-INT_FRAME_SIZE
-	beq-	1f
 	ld	r1,PACAKSAVE(r13)
-1:	std	r10,0(r1)
+	std	r10,0(r1)
 	std	r11,_NIP(r1)
 	std	r12,_MSR(r1)
 	std	r0,GPR0(r1)
 	std	r10,GPR1(r1)
-	beq	2f			/* if from kernel mode */
 #ifdef CONFIG_PPC_FSL_BOOK3E
 START_BTB_FLUSH_SECTION
 	BTB_FLUSH(r10)
 END_BTB_FLUSH_SECTION
 #endif
 	ACCOUNT_CPU_USER_ENTRY(r13, r10, r11)
-2:	std	r2,GPR2(r1)
+	std	r2,GPR2(r1)
 	std	r3,GPR3(r1)
 	mfcr	r2
 	std	r4,GPR4(r1)
@@ -122,14 +118,13 @@ END_BTB_FLUSH_SECTION
 
 #if defined(CONFIG_VIRT_CPU_ACCOUNTING_NATIVE) && defined(CONFIG_PPC_SPLPAR)
 BEGIN_FW_FTR_SECTION
-	beq	33f
-	/* if from user, see if there are any DTL entries to process */
+	/* see if there are any DTL entries to process */
 	ld	r10,PACALPPACAPTR(r13)	/* get ptr to VPA */
 	ld	r11,PACA_DTL_RIDX(r13)	/* get log read index */
 	addi	r10,r10,LPPACA_DTLIDX
 	LDX_BE	r10,0,r10		/* get log write index */
-	cmpd	cr1,r11,r10
-	beq+	cr1,33f
+	cmpd	r11,r10
+	beq+	33f
 	bl	accumulate_stolen_time
 	REST_GPR(0,r1)
 	REST_4GPRS(3,r1)
@@ -203,6 +198,7 @@ system_call:			/* label this so stack traces look sane */
 	mtctr   r12
 	bctrl			/* Call handler */
 
+	/* syscall_exit can exit to kernel mode, via ret_from_kernel_thread */
 .Lsyscall_exit:
 	std	r3,RESULT(r1)
 
@@ -216,11 +212,6 @@ system_call:			/* label this so stack traces look sane */
 	ld	r12, PACA_THREAD_INFO(r13)
 
 	ld	r8,_MSR(r1)
-#ifdef CONFIG_PPC_BOOK3S
-	/* No MSR:RI on BookE */
-	andi.	r10,r8,MSR_RI
-	beq-	.Lunrecov_restore
-#endif
 
 /*
  * This is a few instructions into the actual syscall exit path (which actually

arch/powerpc/kernel/exceptions-64s.S

@@ -1522,8 +1522,6 @@ EXC_COMMON(trap_0b_common, 0xb00, unknown_exception)
  * system call / hypercall (0xc00, 0x4c00)
  *
  * The system call exception is invoked with "sc 0" and does not alter HV bit.
- * There is support for kernel code to invoke system calls but there are no
- * in-tree users.
  *
  * The hypercall is invoked with "sc 1" and sets HV=1.
  *