KVM: x86/xen: Enforce 4-byte alignment of vcpu_info registration

Xen's map_guest_area() enforces that vcpu_info is aligned to
sizeof(xen_ulong_t). KVM has no such check, allowing a guest to
register vcpu_info at an arbitrary byte alignment.

Enforce unconditional 4-byte alignment regardless of the current
shinfo mode. This is sufficient because subsequent commits ensure
that all locked atomic operations on vcpu_info fields use at most
32-bit accesses. Return -ENXIO on failure, matching Xen's
map_guest_area() behaviour for unaligned requests.

Originally observed in review of an unrelated patch:
https://lore.kernel.org/all/20260604193554.1BA311F00893@smtp.kernel.org/

Cc: stable@vger.kernel.org
Fixes: 73e69a86347a ("KVM: x86/xen: register vcpu info")
Reported-by: sashiko-bot@kernel.org
Closes: https://lore.kernel.org/all/20260604193554.1BA311F00893@smtp.kernel.org
Assisted-by: Kiro:claude-opus-4.6-1m
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Link: https://patch.msgid.link/20260831213632.81023-7-dwmw2@infradead.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

0 files changed, 0 insertions, 0 deletions