powerpc/64s: remove KVM SKIP test from instruction breakpoint handler

The code being executed in KVM_GUEST_MODE_SKIP is hypervisor code with
MSR[IR]=0, so the faults of concern are the d-side ones caused by access
to guest context by the hypervisor.

Instruction breakpoint interrupts are not a concern here. It's unlikely
any good would come of causing breaks in this code, but skipping the
instruction that caused it won't help matters (e.g., skip the mtmsr that
sets MSR[DR]=0 or clears KVM_GUEST_MODE_SKIP).

 [Paul notes: "the 0x1300 interrupt was dropped from the architecture a
  long time ago and is not generated by P7, P8, P9 or P10." So add a
  comment about this in the handler code while we're here. ]
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>
Acked-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210412014845.1517916-11-npiggin@gmail.com

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

@@ -2549,11 +2549,16 @@ EXC_REAL_NONE(0x1200, 0x100)
 EXC_VIRT_NONE(0x5200, 0x100)
 #endif
 
-
+/**
+ * Interrupt 0x1300 - Instruction Address Breakpoint Interrupt.
+ * This has been removed from the ISA before 2.01, which is the earliest
+ * 64-bit BookS ISA supported, however the G5 / 970 implements this
+ * interrupt with a non-architected feature available through the support
+ * processor interface.
+ */
 INT_DEFINE_BEGIN(instruction_breakpoint)
 	IVEC=0x1300
 #ifdef CONFIG_KVM_BOOK3S_PR_POSSIBLE
-	IKVM_SKIP=1
 	IKVM_REAL=1
 #endif
 INT_DEFINE_END(instruction_breakpoint)