ANDROID: KVM: arm64: Add serial framework for pKVM

Debugging a hypervisor tends to be trickier than normal system code
such as the kernel. The lack of availability of a UART framework is a
significant contributor to that. In order to address this, introduce a
framework allowing to load serial drivers into the hypervisor.

Bug: 244543039
Bug: 244373730
Change-Id: I2e7a1fd9abc9d5aa9d95f1d271a997d54a8fd582
Signed-off-by: Quentin Perret <qperret@google.com>

arch/arm64/include/asm/kvm_pkvm_module.h

@@ -10,6 +10,7 @@ struct pkvm_module_ops {
 	int (*create_private_mapping)(phys_addr_t phys, size_t size,
 				      enum kvm_pgtable_prot prot,
 				      unsigned long *haddr);
+	int (*register_serial_driver)(void (*hyp_putc_cb)(char));
 };
 
 struct pkvm_module_section {

arch/arm64/kvm/hyp/include/nvhe/serial.h

@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef __ARM64_KVM_NVHE_SERIAL_H__
+#define __ARM64_KVM_NVHE_SERIAL_H__
+
+void hyp_puts(const char *s);
+void hyp_putx64(u64 x);
+void hyp_putc(char c);
+int __pkvm_register_serial_driver(void (*driver_cb)(char));
+
+#endif

arch/arm64/kvm/hyp/nvhe/Makefile

@@ -5,7 +5,8 @@ lib-objs := $(addprefix ../../../lib/, $(lib-objs))
 
 hyp-obj-y := timer-sr.o sysreg-sr.o debug-sr.o switch.o tlb.o hyp-init.o host.o \
 	 hyp-main.o hyp-smp.o psci-relay.o early_alloc.o page_alloc.o \
-	 cache.o setup.o mm.o mem_protect.o sys_regs.o pkvm.o stacktrace.o ffa.o iommu.o
+	 cache.o setup.o mm.o mem_protect.o sys_regs.o pkvm.o stacktrace.o ffa.o iommu.o \
+	 serial.o
 hyp-obj-y += ../vgic-v3-sr.o ../aarch32.o ../vgic-v2-cpuif-proxy.o ../entry.o \
 	 ../fpsimd.o ../hyp-entry.o ../exception.o ../pgtable.o
 hyp-obj-$(CONFIG_DEBUG_LIST) += list_debug.o

arch/arm64/kvm/hyp/nvhe/modules.c

@@ -7,9 +7,11 @@
 
 #include <nvhe/modules.h>
 #include <nvhe/mm.h>
+#include <nvhe/serial.h>
 
 const struct pkvm_module_ops module_ops = {
 	.create_private_mapping = __pkvm_create_private_mapping,
+	.register_serial_driver = __pkvm_register_serial_driver,
 };
 
 int __pkvm_init_module(void *module_init)

arch/arm64/kvm/hyp/nvhe/serial.c

@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2022 - Google LLC
+ */
+
+#include <nvhe/pkvm.h>
+#include <nvhe/spinlock.h>
+
+static void (*__hyp_putc)(char c);
+
+static inline void __hyp_putx4(unsigned int x)
+{
+	x &= 0xf;
+	if (x <= 9)
+		x += '0';
+	else
+		x += ('a' - 0xa);
+
+	__hyp_putc(x);
+}
+
+static inline void __hyp_putx4n(unsigned long x, int n)
+{
+	int i = n >> 2;
+
+	__hyp_putc('0');
+	__hyp_putc('x');
+
+	while (i--)
+		__hyp_putx4(x >> (4 * i));
+
+	__hyp_putc('\n');
+	__hyp_putc('\r');
+}
+
+static inline bool hyp_serial_enabled(void)
+{
+	return !!READ_ONCE(__hyp_putc);
+}
+
+void hyp_puts(const char *s)
+{
+	if (!hyp_serial_enabled())
+		return;
+
+	while (*s)
+		__hyp_putc(*s++);
+
+	__hyp_putc('\n');
+	__hyp_putc('\r');
+}
+
+void hyp_putx64(u64 x)
+{
+	if (hyp_serial_enabled())
+		__hyp_putx4n(x, 64);
+}
+
+void hyp_putc(char c)
+{
+	if (hyp_serial_enabled())
+		__hyp_putc(c);
+}
+
+int __pkvm_register_serial_driver(void (*cb)(char))
+{
+	return cmpxchg(&__hyp_putc, NULL, cb) ? -EBUSY : 0;
+}

arch/arm64/kvm/hyp/nvhe/setup.c

@@ -18,6 +18,7 @@
 #include <nvhe/mem_protect.h>
 #include <nvhe/mm.h>
 #include <nvhe/pkvm.h>
+#include <nvhe/serial.h>
 #include <nvhe/trap_handler.h>
 
 unsigned long hyp_nr_cpus;