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, 124 insertions, 0 deletions

diff --git a/include/drm/spsc_queue.h b/include/drm/spsc_queue.h
new file mode 100644
index 000000000..ee9df8cc6
--- /dev/null
+++ b/include/drm/spsc_queue.h
@@ -0,0 +1,124 @@
+/*
+ * Copyright 2017 Advanced Micro Devices, Inc.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE COPYRIGHT HOLDER(S) OR AUTHOR(S) BE LIABLE FOR ANY CLAIM, DAMAGES OR
+ * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+ * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ * OTHER DEALINGS IN THE SOFTWARE.
+ *
+ */
+
+#ifndef DRM_SCHEDULER_SPSC_QUEUE_H_
+#define DRM_SCHEDULER_SPSC_QUEUE_H_
+
+#include <linux/atomic.h>
+#include <linux/preempt.h>
+
+/** SPSC lockless queue */
+
+struct spsc_node {
+
+	/* Stores spsc_node* */
+	struct spsc_node *next;
+};
+
+struct spsc_queue {
+
+	 struct spsc_node *head;
+
+	/* atomic pointer to struct spsc_node* */
+	atomic_long_t tail;
+
+	atomic_t job_count;
+};
+
+static inline void spsc_queue_init(struct spsc_queue *queue)
+{
+	queue->head = NULL;
+	atomic_long_set(&queue->tail, (long)&queue->head);
+	atomic_set(&queue->job_count, 0);
+}
+
+static inline struct spsc_node *spsc_queue_peek(struct spsc_queue *queue)
+{
+	return queue->head;
+}
+
+static inline int spsc_queue_count(struct spsc_queue *queue)
+{
+	return atomic_read(&queue->job_count);
+}
+
+static inline bool spsc_queue_push(struct spsc_queue *queue, struct spsc_node *node)
+{
+	struct spsc_node **tail;
+
+	node->next = NULL;
+
+	preempt_disable();
+
+	atomic_inc(&queue->job_count);
+	smp_mb__after_atomic();
+
+	tail = (struct spsc_node **)atomic_long_xchg(&queue->tail, (long)&node->next);
+	WRITE_ONCE(*tail, node);
+
+	/*
+	 * In case of first element verify new node will be visible to the consumer
+	 * thread when we ping the kernel thread that there is new work to do.
+	 */
+	smp_wmb();
+
+	preempt_enable();
+
+	return tail == &queue->head;
+}
+
+
+static inline struct spsc_node *spsc_queue_pop(struct spsc_queue *queue)
+{
+	struct spsc_node *next, *node;
+
+	/* Verify reading from memory and not the cache */
+	smp_rmb();
+
+	node = READ_ONCE(queue->head);
+
+	if (!node)
+		return NULL;
+
+	next = READ_ONCE(node->next);
+	WRITE_ONCE(queue->head, next);
+
+	if (unlikely(!next)) {
+		/* slowpath for the last element in the queue */
+
+		if (atomic_long_cmpxchg(&queue->tail,
+				(long)&node->next, (long) &queue->head) != (long)&node->next) {
+			/* Updating tail failed wait for new next to appear */
+			do {
+				smp_rmb();
+			} while (unlikely(!(queue->head = READ_ONCE(node->next))));
+		}
+	}
+
+	atomic_dec(&queue->job_count);
+	return node;
+}
+
+
+
+#endif /* DRM_SCHEDULER_SPSC_QUEUE_H_ */