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, 73 insertions, 0 deletions

diff --git a/tools/include/uapi/README b/tools/include/uapi/README
new file mode 100644
index 000000000..7147b1b2c
--- /dev/null
+++ b/tools/include/uapi/README
@@ -0,0 +1,73 @@
+Why we want a copy of kernel headers in tools?
+==============================================
+
+There used to be no copies, with tools/ code using kernel headers
+directly. From time to time tools/perf/ broke due to legitimate kernel
+hacking. At some point Linus complained about such direct usage. Then we
+adopted the current model.
+
+The way these headers are used in perf are not restricted to just
+including them to compile something.
+
+There are sometimes used in scripts that convert defines into string
+tables, etc, so some change may break one of these scripts, or new MSRs
+may use some different #define pattern, etc.
+
+E.g.:
+
+  $ ls -1 tools/perf/trace/beauty/*.sh | head -5
+  tools/perf/trace/beauty/arch_errno_names.sh
+  tools/perf/trace/beauty/drm_ioctl.sh
+  tools/perf/trace/beauty/fadvise.sh
+  tools/perf/trace/beauty/fsconfig.sh
+  tools/perf/trace/beauty/fsmount.sh
+  $
+  $ tools/perf/trace/beauty/fadvise.sh
+  static const char *fadvise_advices[] = {
+        [0] = "NORMAL",
+        [1] = "RANDOM",
+        [2] = "SEQUENTIAL",
+        [3] = "WILLNEED",
+        [4] = "DONTNEED",
+        [5] = "NOREUSE",
+  };
+  $
+
+The tools/perf/check-headers.sh script, part of the tools/ build
+process, points out changes in the original files.
+
+So its important not to touch the copies in tools/ when doing changes in
+the original kernel headers, that will be done later, when
+check-headers.sh inform about the change to the perf tools hackers.
+
+Another explanation from Ingo Molnar:
+It's better than all the alternatives we tried so far:
+
+ - Symbolic links and direct #includes: this was the original approach but
+   was pushed back on from the kernel side, when tooling modified the
+   headers and broke them accidentally for kernel builds.
+
+ - Duplicate self-defined ABI headers like glibc: double the maintenance
+   burden, double the chance for mistakes, plus there's no tech-driven
+   notification mechanism to look at new kernel side changes.
+
+What we are doing now is a third option:
+
+ - A software-enforced copy-on-write mechanism of kernel headers to
+   tooling, driven by non-fatal warnings on the tooling side build when
+   kernel headers get modified:
+
+    Warning: Kernel ABI header differences:
+      diff -u tools/include/uapi/drm/i915_drm.h include/uapi/drm/i915_drm.h
+      diff -u tools/include/uapi/linux/fs.h include/uapi/linux/fs.h
+      diff -u tools/include/uapi/linux/kvm.h include/uapi/linux/kvm.h
+      ...
+
+   The tooling policy is to always pick up the kernel side headers as-is,
+   and integate them into the tooling build. The warnings above serve as a
+   notification to tooling maintainers that there's changes on the kernel
+   side.
+
+We've been using this for many years now, and it might seem hacky, but
+works surprisingly well.
+