MIPS: KVM: Don't unwind PC when emulating CACHE

When a CACHE instruction is emulated by kvm_mips_emulate_cache(), the PC
is first updated to point to the next instruction, and afterwards it
falls through the "dont_update_pc" label, which rewinds the PC back to
its original address.

This works when dynamic translation of emulated instructions is enabled,
since the CACHE instruction is replaced with a SYNCI which works without
trapping, however when dynamic translation is disabled the guest hangs
on CACHE instructions as they always trap and are never stepped over.

Roughly swap the meanings of the "done" and "dont_update_pc" to match
kvm_mips_emulate_CP0(), so that "done" will roll back the PC on failure,
and "dont_update_pc" won't change PC at all (for the sake of exceptions
that have already modified the PC).
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: kvm@vger.kernel.org
Cc: linux-mips@linux-mips.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/mips/kvm/emulate.c

@@ -1666,7 +1666,7 @@ enum emulation_result kvm_mips_emulate_cache(uint32_t inst, uint32_t *opc,
 			cache, op, base, arch->gprs[base], offset);
 		er = EMULATE_FAIL;
 		preempt_enable();
-		goto dont_update_pc;
+		goto done;
 
 	}
 
@@ -1694,16 +1694,20 @@ enum emulation_result kvm_mips_emulate_cache(uint32_t inst, uint32_t *opc,
 		kvm_err("NO-OP CACHE (cache: %#x, op: %#x, base[%d]: %#lx, offset: %#x\n",
 			cache, op, base, arch->gprs[base], offset);
 		er = EMULATE_FAIL;
-		preempt_enable();
-		goto dont_update_pc;
 	}
 
 	preempt_enable();
+done:
+	/* Rollback PC only if emulation was unsuccessful */
+	if (er == EMULATE_FAIL)
+		vcpu->arch.pc = curr_pc;
 
 dont_update_pc:
-	/* Rollback PC */
-	vcpu->arch.pc = curr_pc;
-done:
+	/*
+	 * This is for exceptions whose emulation updates the PC, so do not
+	 * overwrite the PC under any circumstances
+	 */
+
 	return er;
 }