vmm.c carries the confidential-VM detection used by sd-boot/sd-stub.
Its detect_tdx() had the same dead guard as the userspace copy: it
gated the 0x21 read on CPUID_GET_HIGHEST_FUNCTION (0x80000000, the
extended max function), which is always >= 0x80000000, so the guard
never held.
Mirror the userspace fix: read leaf 0x21 directly and rely on the
IntelTDX signature, matching the kernel. An out-of-range CPUID leaf
returns the highest basic leaf's data (no fault), and 0x21 is a
synthetic TDX leaf whose presence need not be reflected in the max
basic function, so it must not be gated on it.
Ref: Linux 59bd54a84d15 ("x86/tdx: Detect running as a TDX guest in
early boot"), arch/x86/coco/tdx/tdx.c:1119 (tdx_early_init()).
Signed-off-by: Paul Meyer <katexochen0@gmail.com>
Paul Meyer
7c433f3945