target/i386/sev: Add kernel hashes only if sev-guest.kernel-hashes=on

Commit cff03145ed ("sev/i386: Introduce sev_add_kernel_loader_hashes
for measured linux boot", 2021-09-30) introduced measured direct boot
with -kernel, using an OVMF-designated hashes table which QEMU fills.

However, if OVMF doesn't designate such an area, QEMU would completely
abort the VM launch.  This breaks launching with -kernel using older
OVMF images which don't publish the SEV_HASH_TABLE_RV_GUID.

Fix that so QEMU will only look for the hashes table if the sev-guest
kernel-hashes option is set to on.  Otherwise, QEMU won't look for the
designated area in OVMF and won't fill that area.

To enable addition of kernel hashes, launch the guest with:

    -object sev-guest,...,kernel-hashes=on

Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
Acked-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

target/i386/sev.c

@@ -1223,6 +1223,14 @@ bool sev_add_kernel_loader_hashes(SevKernelLoaderContext *ctx, Error **errp)
     size_t hash_len = HASH_SIZE;
     int aligned_len;
 
+    /*
+     * Only add the kernel hashes if the sev-guest configuration explicitly
+     * stated kernel-hashes=on.
+     */
+    if (!sev_guest->kernel_hashes) {
+        return false;
+    }
+
     if (!pc_system_ovmf_table_find(SEV_HASH_TABLE_RV_GUID, &data, NULL)) {
         error_setg(errp, "SEV: kernel specified but OVMF has no hash table guid");
         return false;