KVM: SEV: Do cache maintenance on the source VM during intra-host migrationgrafted

Manually perform cache maintenance on the source VM during intra-host
migration to ensure no stale data is left in CPU caches after the VM is
destroyed.  Because the source VM is "converted" to a non-SEV VM, KVM's
memory reclaim flows won't trigger cache maintenance, e.g. when all guest
memory is reclaimed in response to detaching from the mmu_notifier.

Note, relying on the destination VM to do cache maintenance isn't an option
as KVM doesn't require identical guest memory configurations, i.e. the
source VM may have access to memory that the destination VM does not.
Enforcing equivalent memory configurations is infeasible, as it would
require a *deep* comparison of memslots, e.g. to verify that not only are
the memslot identical, but what the memslots point at is also identical.

Fixes: b56639318bb2 ("KVM: SEV: Add support for SEV intra host migration")
Cc: stable@vger.kernel.org
Reported-by: Stefan Teodorescu <fane@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260923163721.1584779-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 163 insertions, 0 deletions

diff --git a/lib/lockref.c b/lib/lockref.c
new file mode 100644
index 000000000..9b3dd688d
--- /dev/null
+++ b/lib/lockref.c
@@ -0,0 +1,163 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/export.h>
+#include <linux/lockref.h>
+
+#if USE_CMPXCHG_LOCKREF
+
+/*
+ * Note that the "cmpxchg()" reloads the "old" value for the
+ * failure case.
+ */
+#define CMPXCHG_LOOP(CODE, SUCCESS) do {					\
+	int retry = 100;							\
+	struct lockref old;							\
+	BUILD_BUG_ON(sizeof(old) != 8);						\
+	old.lock_count = READ_ONCE(lockref->lock_count);			\
+	while (likely(arch_spin_value_unlocked(old.lock.rlock.raw_lock))) {  	\
+		struct lockref new = old;					\
+		CODE								\
+		if (likely(try_cmpxchg64_relaxed(&lockref->lock_count,		\
+						 &old.lock_count,		\
+						 new.lock_count))) {		\
+			SUCCESS;						\
+		}								\
+		if (!--retry)							\
+			break;							\
+	}									\
+} while (0)
+
+#else
+
+#define CMPXCHG_LOOP(CODE, SUCCESS) do { } while (0)
+
+#endif
+
+/**
+ * lockref_get - Increments reference count unconditionally
+ * @lockref: pointer to lockref structure
+ *
+ * This operation is only valid if you already hold a reference
+ * to the object, so you know the count cannot be zero.
+ */
+void lockref_get(struct lockref *lockref)
+{
+	CMPXCHG_LOOP(
+		new.count++;
+	,
+		return;
+	);
+
+	spin_lock(&lockref->lock);
+	lockref->count++;
+	spin_unlock(&lockref->lock);
+}
+EXPORT_SYMBOL(lockref_get);
+
+/**
+ * lockref_get_not_zero - Increments count unless the count is 0 or dead
+ * @lockref: pointer to lockref structure
+ * Return: 1 if count updated successfully or 0 if count was zero
+ */
+bool lockref_get_not_zero(struct lockref *lockref)
+{
+	bool retval = false;
+
+	CMPXCHG_LOOP(
+		new.count++;
+		if (old.count <= 0)
+			return false;
+	,
+		return true;
+	);
+
+	spin_lock(&lockref->lock);
+	if (lockref->count > 0) {
+		lockref->count++;
+		retval = true;
+	}
+	spin_unlock(&lockref->lock);
+	return retval;
+}
+EXPORT_SYMBOL(lockref_get_not_zero);
+
+/**
+ * lockref_put_return - Decrement reference count if possible
+ * @lockref: pointer to lockref structure
+ *
+ * Decrement the reference count and return the new value.
+ * If the lockref was dead or locked, return -1.
+ */
+int lockref_put_return(struct lockref *lockref)
+{
+	CMPXCHG_LOOP(
+		new.count--;
+		if (old.count <= 0)
+			return -1;
+	,
+		return new.count;
+	);
+	return -1;
+}
+EXPORT_SYMBOL(lockref_put_return);
+
+/**
+ * lockref_put_or_lock - decrements count unless count <= 1 before decrement
+ * @lockref: pointer to lockref structure
+ * Return: 1 if count updated successfully or 0 if count <= 1 and lock taken
+ */
+bool lockref_put_or_lock(struct lockref *lockref)
+{
+	CMPXCHG_LOOP(
+		new.count--;
+		if (old.count <= 1)
+			break;
+	,
+		return true;
+	);
+
+	spin_lock(&lockref->lock);
+	if (lockref->count <= 1)
+		return false;
+	lockref->count--;
+	spin_unlock(&lockref->lock);
+	return true;
+}
+EXPORT_SYMBOL(lockref_put_or_lock);
+
+/**
+ * lockref_mark_dead - mark lockref dead
+ * @lockref: pointer to lockref structure
+ */
+void lockref_mark_dead(struct lockref *lockref)
+{
+	assert_spin_locked(&lockref->lock);
+	lockref->count = __LOCKREF_DEAD_VAL;
+}
+EXPORT_SYMBOL(lockref_mark_dead);
+
+/**
+ * lockref_get_not_dead - Increments count unless the ref is dead
+ * @lockref: pointer to lockref structure
+ * Return: 1 if count updated successfully or 0 if lockref was dead
+ */
+bool lockref_get_not_dead(struct lockref *lockref)
+{
+	bool retval = false;
+
+	CMPXCHG_LOOP(
+		new.count++;
+		if (old.count < 0)
+			return false;
+	,
+		return true;
+	);
+
+	spin_lock(&lockref->lock);
+	if (lockref->count >= 0) {
+		lockref->count++;
+		retval = true;
+	}
+	spin_unlock(&lockref->lock);
+	return retval;
+}
+EXPORT_SYMBOL(lockref_get_not_dead);