core: Add hypercall to write to debug console

In order to debug non-root cell boot-strap procedures, it can be helpful
to have a simple and reliable channel for printing messages, and that
independently of passing physical UARTs through or establishing virtual
networks. Such a channel can easily provided via a hypervisor that
writes to the hypervisor's debug console.

However, only invocations from cells which have the explicit permission
via the configuration flag JAILHOUSE_CELL_DEBUG_CONSOLE are executed.
The default remains off, and the administrator is expected to grant this
permission only temporarily while debugging a specific cell.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

Documentation/hypervisor-interfaces.txt

@@ -255,6 +255,20 @@ Return code: Requested value (>=0) or negative error code
         -EINVAL (-22) - invalid CPU ID
 
 
+Hypercall "Debug Console putc" (code 8)
+- - - - - - - - - - - - - - - - - - - -
+
+Write a single character to the hypervisor's debug console.
+
+Arguments: 1. character to write
+
+Return code: 0 on success, negative error code otherwise
+
+    Possible errors are:
+        -EPERM  (-1)  - cell lacks JAILHOUSE_CELL_DEBUG_CONSOLE flag in its
+                        configuration
+
+
 Communication Region
 --------------------
 

hypervisor/control.c

@@ -791,6 +791,12 @@ long hypercall(unsigned long code, unsigned long arg1, unsigned long arg2)
 		return cell_get_state(cpu_data, arg1);
 	case JAILHOUSE_HC_CPU_GET_INFO:
 		return cpu_get_info(cpu_data, arg1, arg2);
+	case JAILHOUSE_HC_DEBUG_CONSOLE_PUTC:
+		if (!(cpu_data->cell->config->flags &
+		      JAILHOUSE_CELL_DEBUG_CONSOLE))
+			return -EPERM;
+		printk("%c", (char)arg1);
+		return 0;
 	default:
 		return -ENOSYS;
 	}

hypervisor/include/jailhouse/cell-config.h

@@ -42,6 +42,7 @@
 #define JAILHOUSE_CELL_NAME_MAXLEN	31
 
 #define JAILHOUSE_CELL_PASSIVE_COMMREG	0x00000001
+#define JAILHOUSE_CELL_DEBUG_CONSOLE	0x00000002
 
 #define JAILHOUSE_CELL_DESC_SIGNATURE	"JAILCELL"
 

hypervisor/include/jailhouse/hypercall.h

@@ -47,6 +47,7 @@
 #define JAILHOUSE_HC_HYPERVISOR_GET_INFO	5
 #define JAILHOUSE_HC_CELL_GET_STATE		6
 #define JAILHOUSE_HC_CPU_GET_INFO		7
+#define JAILHOUSE_HC_DEBUG_CONSOLE_PUTC		8
 
 /* Hypervisor information type */
 #define JAILHOUSE_INFO_MEM_POOL_SIZE		0