[PATCH v3 26/31] KVM: arm64: Add atomics-based checking refcount implementation at EL2

[Fuad Tabba <tabba@google.com>]

From: Will Deacon <will@kernel.org>

The current nVHE refcount implementation at EL2 uses a simple spinlock
to serialise accesses. Although this works, it forces serialisation in
places where it is not necessary, so introduce a simple atomics-based
refcount implementation instead.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Fuad Tabba <tabba@google.com>
---
 arch/arm64/kvm/hyp/include/nvhe/refcount.h | 76 ++++++++++++++++++++++
 1 file changed, 76 insertions(+)
 create mode 100644 arch/arm64/kvm/hyp/include/nvhe/refcount.h

diff --git a/arch/arm64/kvm/hyp/include/nvhe/refcount.h b/arch/arm64/kvm/hyp/include/nvhe/refcount.h
new file mode 100644
index 000000000000..bf6bd4ef89bf
--- /dev/null
+++ b/arch/arm64/kvm/hyp/include/nvhe/refcount.h
@@ -0,0 +1,76 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Atomics-based checking refcount implementation.
+ * Copyright (C) 2023 Google LLC
+ * Author: Will Deacon <will@kernel.org>
+ */
+#ifndef __ARM64_KVM_NVHE_REFCOUNT_H__
+#define __ARM64_KVM_NVHE_REFCOUNT_H__
+
+#include <asm/lse.h>
+
+static inline s16 __ll_sc_refcount_fetch_add_16(u16 *refcount, s16 addend)
+{
+	u16 new;
+	u32 flag;
+
+	asm volatile(
+	"	prfm	pstl1strm, %[refcount]\n"
+	"1:	ldxrh	%w[new], %[refcount]\n"
+	"	add	%w[new], %w[new], %w[addend]\n"
+	"	stxrh	%w[flag], %w[new], %[refcount]\n"
+	"	cbnz	%w[flag], 1b"
+	: [refcount] "+Q" (*refcount),
+	  [new] "=&r" (new),
+	  [flag] "=&r" (flag)
+	: [addend] "Ir" (addend));
+
+	return new;
+}
+
+#ifdef CONFIG_ARM64_LSE_ATOMICS
+
+static inline s16 __lse_refcount_fetch_add_16(u16 *refcount, s16 addend)
+{
+	s16 old;
+
+	asm volatile(__LSE_PREAMBLE
+	"	ldaddh	%w[addend], %w[old], %[refcount]"
+	: [refcount] "+Q" (*refcount),
+	  [old] "=r" (old)
+	: [addend] "r" (addend));
+
+	return old + addend;
+}
+
+#endif /* CONFIG_ARM64_LSE_ATOMICS */
+
+static inline u64 __hyp_refcount_fetch_add(void *refcount, const size_t size,
+					   const s64 addend)
+{
+	s64 new;
+
+	switch (size) {
+	case 2:
+		new = __lse_ll_sc_body(refcount_fetch_add_16, refcount, addend);
+		break;
+	default:
+		BUILD_BUG_ON_MSG(1, "Unsupported refcount size");
+		unreachable();
+	}
+
+	BUG_ON(new < 0);
+	return new;
+}
+
+
+#define hyp_refcount_inc(r)	__hyp_refcount_fetch_add(&(r), sizeof(r), 1)
+#define hyp_refcount_dec(r)	__hyp_refcount_fetch_add(&(r), sizeof(r), -1)
+#define hyp_refcount_get(r)	READ_ONCE(r)
+#define hyp_refcount_set(r, v)	do {			\
+	typeof(r) *__rp = &(r);				\
+	WARN_ON(hyp_refcount_get(*__rp));		\
+	WRITE_ONCE(*__rp, v);				\
+} while (0)
+
+#endif /* __ARM64_KVM_NVHE_REFCOUNT_H__ */
-- 
2.44.0.769.g3c40516874-goog