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, 37 insertions, 0 deletions

diff --git a/Documentation/ABI/removed/sysfs-mce b/Documentation/ABI/removed/sysfs-mce
new file mode 100644
index 000000000..ef5dd2a80
--- /dev/null
+++ b/Documentation/ABI/removed/sysfs-mce
@@ -0,0 +1,37 @@
+What:		/sys/devices/system/machinecheck/machinecheckX/tolerant
+Contact:	Borislav Petkov <bp@suse.de>
+Date:		Dec, 2021
+Description:
+		Unused and obsolete after the advent of recoverable machine
+		checks (see last sentence below) and those are present since
+		2010 (Nehalem).
+
+		Original description:
+
+		The entries appear for each CPU, but they are truly shared
+		between all CPUs.
+
+		Tolerance level. When a machine check exception occurs for a
+		non corrected machine check the kernel can take different
+		actions.
+
+		Since machine check exceptions can happen any time it is
+		sometimes risky for the kernel to kill a process because it
+		defies normal kernel locking rules. The tolerance level
+		configures how hard the kernel tries to recover even at some
+		risk of	deadlock. Higher tolerant values trade potentially
+		better uptime with the risk of a crash or even corruption
+		(for tolerant >= 3).
+
+		==  ===========================================================
+		 0  always panic on uncorrected errors, log corrected errors
+		 1  panic or SIGBUS on uncorrected errors, log corrected errors
+		 2  SIGBUS or log uncorrected errors, log corrected errors
+		 3  never panic or SIGBUS, log all errors (for testing only)
+		==  ===========================================================
+
+		Default: 1
+
+		Note this only makes a difference if the CPU allows recovery
+		from a machine check exception. Current x86 CPUs generally
+		do not.