target/ppc: Fix sc instruction handling of LEV field

The top bits of the LEV field of the sc instruction are to be treated as
as a reserved field rather than a reserved value, meaning LEV is
effectively the bottom bit. LEV=0xF should be treated as LEV=1 and be
a hypercall, for example.

This changes the instruction execution to just set lev from the low bit
of the field. Processors which don't support the LEV field will continue
to ignore it.

ISA v3.1 defines LEV to be 2 bits, in order to add the 'sc 2' ultracall
instruction. TCG does not support Ultravisor, so don't worry about
that bit.

Suggested-by: "Harsh Prateek Bora" <harshpb@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

target/ppc/translate.c

@@ -4429,7 +4429,12 @@ static void gen_sc(DisasContext *ctx)
 {
     uint32_t lev;
 
-    lev = (ctx->opcode >> 5) & 0x7F;
+    /*
+     * LEV is a 7-bit field, but the top 6 bits are treated as a reserved
+     * field (i.e., ignored). ISA v3.1 changes that to 5 bits, but that is
+     * for Ultravisor which TCG does not support, so just ignore the top 6.
+     */
+    lev = (ctx->opcode >> 5) & 0x1;
     gen_exception_err(ctx, POWERPC_SYSCALL, lev);
 }