[PATCH v1 25/44] KVM: arm64: Introduce hyp_rwlock_t

[Fuad Tabba <tabba@google.com>]

From: Will Deacon <will@kernel.org>

Introduce a simple counter-based rwlock for EL2 which can reduce locking
contention on read-mostly data structures when compared to a spinlock.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Fuad Tabba <tabba@google.com>
---
 arch/arm64/kvm/hyp/include/nvhe/rwlock.h | 129 +++++++++++++++++++++++
 1 file changed, 129 insertions(+)
 create mode 100644 arch/arm64/kvm/hyp/include/nvhe/rwlock.h

diff --git a/arch/arm64/kvm/hyp/include/nvhe/rwlock.h b/arch/arm64/kvm/hyp/include/nvhe/rwlock.h
new file mode 100644
index 000000000000..365084497e59
--- /dev/null
+++ b/arch/arm64/kvm/hyp/include/nvhe/rwlock.h
@@ -0,0 +1,129 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * A stand-alone rwlock implementation for use by the non-VHE KVM
+ * hypervisor code running at EL2. This is *not* a fair lock and is
+ * likely to scale very badly under contention.
+ *
+ * Copyright (C) 2022 Google LLC
+ * Author: Will Deacon <will@kernel.org>
+ *
+ * Heavily based on the implementation removed by 087133ac9076 which was:
+ * Copyright (C) 2012 ARM Ltd.
+ */
+
+#ifndef __ARM64_KVM_NVHE_RWLOCK_H__
+#define __ARM64_KVM_NVHE_RWLOCK_H__
+
+#include <linux/bits.h>
+
+typedef struct {
+	u32	__val;
+} hyp_rwlock_t;
+
+#define __HYP_RWLOCK_INITIALIZER \
+	{ .__val = 0 }
+
+#define __HYP_RWLOCK_UNLOCKED \
+	((hyp_rwlock_t) __HYP_RWLOCK_INITIALIZER)
+
+#define DEFINE_HYP_RWLOCK(x)	hyp_rwlock_t x = __HYP_RWLOCK_UNLOCKED
+
+#define hyp_rwlock_init(l)						\
+do {									\
+	*(l) = __HYP_RWLOCK_UNLOCKED;					\
+} while (0)
+
+#define __HYP_RWLOCK_WRITER_BIT	31
+
+static inline void hyp_write_lock(hyp_rwlock_t *lock)
+{
+	u32 tmp;
+
+	asm volatile(ARM64_LSE_ATOMIC_INSN(
+	/* LL/SC */
+	"	sevl\n"
+	"1:	wfe\n"
+	"2:	ldaxr	%w0, %1\n"
+	"	cbnz	%w0, 1b\n"
+	"	stxr	%w0, %w2, %1\n"
+	"	cbnz	%w0, 2b\n"
+	__nops(1),
+	/* LSE atomics */
+	"1:	mov	%w0, wzr\n"
+	"2:	casa	%w0, %w2, %1\n"
+	"	cbz	%w0, 3f\n"
+	"	ldxr	%w0, %1\n"
+	"	cbz	%w0, 2b\n"
+	"	wfe\n"
+	"	b	1b\n"
+	"3:")
+	: "=&r" (tmp), "+Q" (lock->__val)
+	: "r" (BIT(__HYP_RWLOCK_WRITER_BIT))
+	: "memory");
+}
+
+static inline void hyp_write_unlock(hyp_rwlock_t *lock)
+{
+	asm volatile(ARM64_LSE_ATOMIC_INSN(
+	"	stlr	wzr, %0",
+	"	swpl	wzr, wzr, %0")
+	: "=Q" (lock->__val) :: "memory");
+}
+
+static inline void hyp_read_lock(hyp_rwlock_t *lock)
+{
+	u32 tmp, tmp2;
+
+	asm volatile(
+	"	sevl\n"
+	ARM64_LSE_ATOMIC_INSN(
+	/* LL/SC */
+	"1:	wfe\n"
+	"2:	ldaxr	%w0, %2\n"
+	"	add	%w0, %w0, #1\n"
+	"	tbnz	%w0, %3, 1b\n"
+	"	stxr	%w1, %w0, %2\n"
+	"	cbnz	%w1, 2b\n"
+	__nops(1),
+	/* LSE atomics */
+	"1:	wfe\n"
+	"2:	ldxr	%w0, %2\n"
+	"	adds	%w1, %w0, #1\n"
+	"	tbnz	%w1, %3, 1b\n"
+	"	casa	%w0, %w1, %2\n"
+	"	sbc	%w0, %w1, %w0\n"
+	"	cbnz	%w0, 2b")
+	: "=&r" (tmp), "=&r" (tmp2), "+Q" (lock->__val)
+	: "i" (__HYP_RWLOCK_WRITER_BIT)
+	: "cc", "memory");
+}
+
+static inline void hyp_read_unlock(hyp_rwlock_t *lock)
+{
+	u32 tmp, tmp2;
+
+	asm volatile(ARM64_LSE_ATOMIC_INSN(
+	/* LL/SC */
+	"1:	ldxr	%w0, %2\n"
+	"	sub	%w0, %w0, #1\n"
+	"	stlxr	%w1, %w0, %2\n"
+	"	cbnz	%w1, 1b",
+	/* LSE atomics */
+	"	movn	%w0, #0\n"
+	"	staddl	%w0, %2\n"
+	__nops(2))
+	: "=&r" (tmp), "=&r" (tmp2), "+Q" (lock->__val)
+	:
+	: "memory");
+}
+
+#ifdef CONFIG_NVHE_EL2_DEBUG
+static inline void hyp_assert_write_lock_held(hyp_rwlock_t *lock)
+{
+	BUG_ON(!(READ_ONCE(lock->__val) & BIT(__HYP_RWLOCK_WRITER_BIT)));
+}
+#else
+static inline void hyp_assert_write_lock_held(hyp_rwlock_t *lock) { }
+#endif
+
+#endif	/* __ARM64_KVM_NVHE_RWLOCK_H__ */
-- 
2.44.0.478.gd926399ef9-goog