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, 55 insertions, 0 deletions

diff --git a/Documentation/admin-guide/numastat.rst b/Documentation/admin-guide/numastat.rst
new file mode 100644
index 000000000..08ec2c2bd
--- /dev/null
+++ b/Documentation/admin-guide/numastat.rst
@@ -0,0 +1,55 @@
+===============================
+Numa policy hit/miss statistics
+===============================
+
+/sys/devices/system/node/node*/numastat
+
+All units are pages. Hugepages have separate counters.
+
+The numa_hit, numa_miss and numa_foreign counters reflect how well processes
+are able to allocate memory from nodes they prefer. If they succeed, numa_hit
+is incremented on the preferred node, otherwise numa_foreign is incremented on
+the preferred node and numa_miss on the node where allocation succeeded.
+
+Usually preferred node is the one local to the CPU where the process executes,
+but restrictions such as mempolicies can change that, so there are also two
+counters based on CPU local node. local_node is similar to numa_hit and is
+incremented on allocation from a node by CPU on the same node. other_node is
+similar to numa_miss and is incremented on the node where allocation succeeds
+from a CPU from a different node. Note there is no counter analogical to
+numa_foreign.
+
+In more detail:
+
+=============== ============================================================
+numa_hit	A process wanted to allocate memory from this node,
+		and succeeded.
+
+numa_miss	A process wanted to allocate memory from another node,
+		but ended up with memory from this node.
+
+numa_foreign	A process wanted to allocate on this node,
+		but ended up with memory from another node.
+
+local_node	A process ran on this node's CPU,
+		and got memory from this node.
+
+other_node	A process ran on a different node's CPU
+		and got memory from this node.
+
+interleave_hit 	Interleaving wanted to allocate from this node
+		and succeeded.
+=============== ============================================================
+
+For easier reading you can use the numastat utility from the numactl package
+(http://oss.sgi.com/projects/libnuma/). Note that it only works
+well right now on machines with a small number of CPUs.
+
+Note that on systems with memoryless nodes (where a node has CPUs but no
+memory) the numa_hit, numa_miss and numa_foreign statistics can be skewed
+heavily. In the current kernel implementation, if a process prefers a
+memoryless node (i.e.  because it is running on one of its local CPU), the
+implementation actually treats one of the nearest nodes with memory as the
+preferred node. As a result, such allocation will not increase the numa_foreign
+counter on the memoryless node, and will skew the numa_hit, numa_miss and
+numa_foreign statistics of the nearest node.