rqspinlock: Add basic support for CONFIG_PARAVIRT

We ripped out PV and virtualization related bits from rqspinlock in an
earlier commit, however, a fair lock performs poorly within a virtual
machine when the lock holder is preempted. As such, retain the
virt_spin_lock fallback to test and set lock, but with timeout and
deadlock detection. We can do this by simply depending on the
resilient_tas_spin_lock implementation from the previous patch.

We don't integrate support for CONFIG_PARAVIRT_SPINLOCKS yet, as that
requires more involved algorithmic changes and introduces more
complexity. It can be done when the need arises in the future.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20250316040541.108729-15-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

arch/x86/include/asm/rqspinlock.h

@@ -0,0 +1,33 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_RQSPINLOCK_H
+#define _ASM_X86_RQSPINLOCK_H
+
+#include <asm/paravirt.h>
+
+#ifdef CONFIG_PARAVIRT
+DECLARE_STATIC_KEY_FALSE(virt_spin_lock_key);
+
+#define resilient_virt_spin_lock_enabled resilient_virt_spin_lock_enabled
+static __always_inline bool resilient_virt_spin_lock_enabled(void)
+{
+       return static_branch_likely(&virt_spin_lock_key);
+}
+
+#ifdef CONFIG_QUEUED_SPINLOCKS
+typedef struct qspinlock rqspinlock_t;
+#else
+typedef struct rqspinlock rqspinlock_t;
+#endif
+extern int resilient_tas_spin_lock(rqspinlock_t *lock);
+
+#define resilient_virt_spin_lock resilient_virt_spin_lock
+static inline int resilient_virt_spin_lock(rqspinlock_t *lock)
+{
+	return resilient_tas_spin_lock(lock);
+}
+
+#endif /* CONFIG_PARAVIRT */
+
+#include <asm-generic/rqspinlock.h>
+
+#endif /* _ASM_X86_RQSPINLOCK_H */

include/asm-generic/rqspinlock.h

@@ -35,6 +35,20 @@ extern int resilient_tas_spin_lock(rqspinlock_t *lock);
 extern int resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val);
 #endif
 
+#ifndef resilient_virt_spin_lock_enabled
+static __always_inline bool resilient_virt_spin_lock_enabled(void)
+{
+	return false;
+}
+#endif
+
+#ifndef resilient_virt_spin_lock
+static __always_inline int resilient_virt_spin_lock(rqspinlock_t *lock)
+{
+	return 0;
+}
+#endif
+
 /*
  * Default timeout for waiting loops is 0.25 seconds
  */

kernel/bpf/rqspinlock.c

@@ -352,6 +352,9 @@ int __lockfunc resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val)
 
 	BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
 
+	if (resilient_virt_spin_lock_enabled())
+		return resilient_virt_spin_lock(lock);
+
 	RES_INIT_TIMEOUT(ts);
 
 	/*