arm64: kvm: Add standalone ticket spinlock implementation for use at hyp

We will soon need to synchronise multiple CPUs in the hyp text at EL2.
The qspinlock-based locking used by the host is overkill for this purpose
and relies on the kernel's "percpu" implementation for the MCS nodes.

Implement a simple ticket locking scheme based heavily on the code removed
by commit c1109047 ("arm64: locking: Replace ticket lock implementation
with qspinlock").
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210319100146.1149909-4-qperret@google.com

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

+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * A stand-alone ticket spinlock implementation for use by the non-VHE
+ * KVM hypervisor code running at EL2.
+ *
+ * Copyright (C) 2020 Google LLC
+ * Author: Will Deacon <will@kernel.org>
+ *
+ * Heavily based on the implementation removed by c11090474d70 which was:
+ * Copyright (C) 2012 ARM Ltd.
+ */
+
+#ifndef __ARM64_KVM_NVHE_SPINLOCK_H__
+#define __ARM64_KVM_NVHE_SPINLOCK_H__
+
+#include <asm/alternative.h>
+#include <asm/lse.h>
+
+typedef union hyp_spinlock {
+	u32	__val;
+	struct {
+#ifdef __AARCH64EB__
+		u16 next, owner;
+#else
+		u16 owner, next;
+#endif
+	};
+} hyp_spinlock_t;
+
+#define hyp_spin_lock_init(l)						\
+do {									\
+	*(l) = (hyp_spinlock_t){ .__val = 0 };				\
+} while (0)
+
+static inline void hyp_spin_lock(hyp_spinlock_t *lock)
+{
+	u32 tmp;
+	hyp_spinlock_t lockval, newval;
+
+	asm volatile(
+	/* Atomically increment the next ticket. */
+	ARM64_LSE_ATOMIC_INSN(
+	/* LL/SC */
+"	prfm	pstl1strm, %3\n"
+"1:	ldaxr	%w0, %3\n"
+"	add	%w1, %w0, #(1 << 16)\n"
+"	stxr	%w2, %w1, %3\n"
+"	cbnz	%w2, 1b\n",
+	/* LSE atomics */
+"	mov	%w2, #(1 << 16)\n"
+"	ldadda	%w2, %w0, %3\n"
+	__nops(3))
+
+	/* Did we get the lock? */
+"	eor	%w1, %w0, %w0, ror #16\n"
+"	cbz	%w1, 3f\n"
+	/*
+	 * No: spin on the owner. Send a local event to avoid missing an
+	 * unlock before the exclusive load.
+	 */
+"	sevl\n"
+"2:	wfe\n"
+"	ldaxrh	%w2, %4\n"
+"	eor	%w1, %w2, %w0, lsr #16\n"
+"	cbnz	%w1, 2b\n"
+	/* We got the lock. Critical section starts here. */
+"3:"
+	: "=&r" (lockval), "=&r" (newval), "=&r" (tmp), "+Q" (*lock)
+	: "Q" (lock->owner)
+	: "memory");
+}
+
+static inline void hyp_spin_unlock(hyp_spinlock_t *lock)
+{
+	u64 tmp;
+
+	asm volatile(
+	ARM64_LSE_ATOMIC_INSN(
+	/* LL/SC */
+	"	ldrh	%w1, %0\n"
+	"	add	%w1, %w1, #1\n"
+	"	stlrh	%w1, %0",
+	/* LSE atomics */
+	"	mov	%w1, #1\n"
+	"	staddlh	%w1, %0\n"
+	__nops(1))
+	: "=Q" (lock->owner), "=&r" (tmp)
+	:
+	: "memory");
+}
+
+#endif /* __ARM64_KVM_NVHE_SPINLOCK_H__ */