jailhouse: Add simple debug console via the hypervisor

Jailhouse allows explicitly enabled cells to write character-wise
messages to the hypervisor debug console. Make use of this for a
platform-agnostic boot diagnosis channel, specifically for non-root
cells. This also comes with earlycon support.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
(cherry picked from commit 60685bd589aef4972d20724863079edf2039eaa2)
From http://git.kiszka.org/?p=linux.git;a=shortlog;h=refs/heads/queues/jailhouse

MAINTAINERS

@@ -8761,6 +8761,7 @@ L:	jailhouse-dev@googlegroups.com
 S:	Maintained
 F:	arch/x86/kernel/jailhouse.c
 F:	arch/x86/include/asm/jailhouse_para.h
+F:	drivers/virt/jailhouse_dbgcon.c
 
 JC42.4 TEMPERATURE SENSOR DRIVER
 M:	Guenter Roeck <linux@roeck-us.net>

drivers/virt/Kconfig

@@ -31,5 +31,16 @@ config FSL_HV_MANAGER
           4) A kernel interface for receiving callbacks when a managed
 	     partition shuts down.
 
+config JAILHOUSE_DBGCON
+	tristate "Jailhouse console driver"
+	depends on X86 || ARM || ARM64
+	help
+	  The Jailhouse hypervisor provides a simple write-only console for
+	  debugging the bootstrap process of its cells. This driver registers
+	  a console with the kernel to make use of it.
+
+	  Note that Jailhouse has to be configured to permit a cell the usage
+	  of the console interface.
+
 source "drivers/virt/vboxguest/Kconfig"
 endif

drivers/virt/Makefile

@@ -4,4 +4,5 @@
 #
 
 obj-$(CONFIG_FSL_HV_MANAGER)	+= fsl_hypervisor.o
+obj-$(CONFIG_JAILHOUSE_DBGCON)	+= jailhouse_dbgcon.o
 obj-y				+= vboxguest/

drivers/virt/jailhouse_dbgcon.c

@@ -0,0 +1,103 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Console driver for running over the Jailhouse partitioning hypervisor
+ *
+ * Copyright (c) Siemens AG, 2016-2018
+ *
+ * Authors:
+ *  Jan Kiszka <jan.kiszka@siemens.com>
+ */
+
+#include <linux/console.h>
+#include <linux/hypervisor.h>
+#include <linux/module.h>
+#include <linux/serial_core.h>
+#ifdef CONFIG_X86
+#include <asm/alternative.h>
+#endif
+#ifdef CONFIG_ARM
+#include <asm/opcodes-virt.h>
+#endif
+
+#define JAILHOUSE_HC_DEBUG_CONSOLE_PUTC		8
+
+static void hypervisor_putc(char c)
+{
+#if defined(CONFIG_X86)
+	int result;
+
+	asm volatile(
+		ALTERNATIVE(".byte 0x0f,0x01,0xc1", ".byte 0x0f,0x01,0xd9",
+			    X86_FEATURE_VMMCALL)
+		: "=a" (result)
+		: "a" (JAILHOUSE_HC_DEBUG_CONSOLE_PUTC), "D" (c)
+		: "memory");
+#elif defined(CONFIG_ARM)
+	register u32 num_res asm("r0") = JAILHOUSE_HC_DEBUG_CONSOLE_PUTC;
+	register u32 arg1 asm("r1") = c;
+
+	asm volatile(
+		__HVC(0x4a48)
+		: "=r" (num_res)
+		: "r" (num_res), "r" (arg1)
+		: "memory");
+#elif defined(CONFIG_ARM64)
+	register u64 num_res asm("x0") = JAILHOUSE_HC_DEBUG_CONSOLE_PUTC;
+	register u64 arg1 asm("x1") = c;
+
+	asm volatile(
+		"hvc #0x4a48\n\t"
+		: "=r" (num_res)
+		: "r" (num_res), "r" (arg1)
+		: "memory");
+#else
+#error Unsupported architecture.
+#endif
+}
+
+static void jailhouse_dbgcon_write(struct console *con, const char *s,
+				   unsigned count)
+{
+	while (count > 0) {
+		hypervisor_putc(*s);
+		count--;
+		s++;
+	}
+}
+
+static int __init early_jailhouse_dbgcon_setup(struct earlycon_device *device,
+					       const char *options)
+{
+	device->con->write = jailhouse_dbgcon_write;
+	return 0;
+}
+
+EARLYCON_DECLARE(jailhouse, early_jailhouse_dbgcon_setup);
+
+static struct console jailhouse_dbgcon = {
+	.name = "jailhouse",
+	.write = jailhouse_dbgcon_write,
+	.flags = CON_PRINTBUFFER | CON_ANYTIME,
+	.index = -1,
+};
+
+static int __init jailhouse_dbgcon_init(void)
+{
+	if (!jailhouse_paravirt())
+		return -ENODEV;
+
+	register_console(&jailhouse_dbgcon);
+	return 0;
+}
+
+static void __exit jailhouse_dbgcon_exit(void)
+{
+	unregister_console(&jailhouse_dbgcon);
+}
+
+module_init(jailhouse_dbgcon_init);
+module_exit(jailhouse_dbgcon_exit);
+
+MODULE_LICENSE("GPL v2");
+MODULE_DESCRIPTION("Jailhouse debug console driver");
+MODULE_AUTHOR("Jan Kiszka <jan.kiszka@siemens.com>");