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[RFC] powerpc/emulate_step: Fix kernel crash when VSX is not present

emulate_step() is not checking runtime VSX feature flag before
emulating an instruction. This can cause kernel oops when kernel
is compiled with CONFIG_VSX=y but running on machine where VSX is
not supported or disabled. Ex, while running emulate_step tests on
P6 machine:

  emulate_step_test: lvx            : PASS
  emulate_step_test: stvx           : PASS
  Oops: Exception in kernel mode, sig: 4 [#1]
  NIP [c000000000095c24] .load_vsrn+0x28/0x54
  LR [c000000000094bdc] .emulate_loadstore+0x167c/0x17b0
  Call Trace:
  [c0000000494c3770] [40fe240c7ae147ae] 0x40fe240c7ae147ae (unreliable)
  [c0000000494c37f0] [c000000000094bdc] .emulate_loadstore+0x167c/0x17b0
  [c0000000494c3900] [c000000000094f6c] .emulate_step+0x25c/0x5bc
  [c0000000494c39b0] [c000000000ec90dc] .test_lxvd2x_stxvd2x+0x64/0x154
  [c0000000494c3b90] [c000000000ec9204] .test_emulate_step+0x38/0x4c
  [c0000000494c3c00] [c00000000000de2c] .do_one_initcall+0x5c/0x2c0
  [c0000000494c3cd0] [c000000000eb4bf8] .kernel_init_freeable+0x314/0x4cc
  [c0000000494c3db0] [c00000000000e1e4] .kernel_init+0x24/0x160
  [c0000000494c3e30] [c00000000000bc24] .ret_from_kernel_thread+0x58/0xb4

With fix:
  emulate_step_test: stvx           : PASS
  emulate_step_test: lxvd2x         : FAIL
  emulate_step_test: stxvd2x        : FAIL

Fixes: https://github.com/linuxppc/linux/issues/148

Reported-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxx>
Note: VSX was first introduced on P6. But there are many VSX instructions
      which were introduced in later versions. So ideally, analyse_instr()
      should check for ISA version along with opcode.

 arch/powerpc/lib/sstep.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/powerpc/lib/sstep.c b/arch/powerpc/lib/sstep.c
index 34d68f1b1b40..470942f9c567 100644
--- a/arch/powerpc/lib/sstep.c
+++ b/arch/powerpc/lib/sstep.c
@@ -1173,6 +1173,7 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
 	unsigned long int val, val2;
 	unsigned int mb, me, sh;
 	long ival;
+	int type;
 	op->type = COMPUTE;
@@ -2544,6 +2545,15 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
 #endif /* __powerpc64__ */
+#ifdef CONFIG_VSX
+	type = op->type & INSTR_TYPE_MASK;
+	if ((type == LOAD_VSX || type == STORE_VSX) &&
+	    !cpu_has_feature(CPU_FTR_VSX)) {
+		return -1;
+	}
+#endif /* CONFIG_VSX */
 	return 0;