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, 107 insertions, 0 deletions

diff --git a/include/drm/task_barrier.h b/include/drm/task_barrier.h
new file mode 100644
index 000000000..f6e6ed529
--- /dev/null
+++ b/include/drm/task_barrier.h
@@ -0,0 +1,107 @@
+/*
+ * Copyright 2019 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.
+ *
+ */
+#include <linux/semaphore.h>
+#include <linux/atomic.h>
+
+/*
+ * Reusable 2 PHASE task barrier (rendez-vous point) implementation for N tasks.
+ * Based on the Little book of semaphores - https://greenteapress.com/wp/semaphores/
+ */
+
+
+
+#ifndef DRM_TASK_BARRIER_H_
+#define DRM_TASK_BARRIER_H_
+
+/*
+ * Represents an instance of a task barrier.
+ */
+struct task_barrier {
+	unsigned int n;
+	atomic_t count;
+	struct semaphore enter_turnstile;
+	struct semaphore exit_turnstile;
+};
+
+static inline void task_barrier_signal_turnstile(struct semaphore *turnstile,
+						 unsigned int n)
+{
+	int i;
+
+	for (i = 0 ; i < n; i++)
+		up(turnstile);
+}
+
+static inline void task_barrier_init(struct task_barrier *tb)
+{
+	tb->n = 0;
+	atomic_set(&tb->count, 0);
+	sema_init(&tb->enter_turnstile, 0);
+	sema_init(&tb->exit_turnstile, 0);
+}
+
+static inline void task_barrier_add_task(struct task_barrier *tb)
+{
+	tb->n++;
+}
+
+static inline void task_barrier_rem_task(struct task_barrier *tb)
+{
+	tb->n--;
+}
+
+/*
+ * Lines up all the threads BEFORE the critical point.
+ *
+ * When all thread passed this code the entry barrier is back to locked state.
+ */
+static inline void task_barrier_enter(struct task_barrier *tb)
+{
+	if (atomic_inc_return(&tb->count) == tb->n)
+		task_barrier_signal_turnstile(&tb->enter_turnstile, tb->n);
+
+	down(&tb->enter_turnstile);
+}
+
+/*
+ * Lines up all the threads AFTER the critical point.
+ *
+ * This function is used to avoid any one thread running ahead if the barrier is
+ *  used repeatedly .
+ */
+static inline void task_barrier_exit(struct task_barrier *tb)
+{
+	if (atomic_dec_return(&tb->count) == 0)
+		task_barrier_signal_turnstile(&tb->exit_turnstile, tb->n);
+
+	down(&tb->exit_turnstile);
+}
+
+/* Convinieince function when nothing to be done in between entry and exit */
+static inline void task_barrier_full(struct task_barrier *tb)
+{
+	task_barrier_enter(tb);
+	task_barrier_exit(tb);
+}
+
+#endif