KVM: SEV: Document/enforce page-alignment for KVM_SEV_SNP_LAUNCH_UPDATE

In the past, KVM_SEV_SNP_LAUNCH_UPDATE accepted a non-page-aligned
'uaddr' parameter to copy data from, but continuing to support this with
new functionality like in-place conversion and hugepages in the pipeline
has proven to be more trouble than it is worth, since there are no known
users that have been identified who use a non-page-aligned 'uaddr'
parameter.

Rather than locking guest_memfd into continuing to support this, go
ahead and document page-alignment as a requirement and begin enforcing
this in the handling function.

Reviewed-by: Vishal Annapurve <vannapurve@google.com>
Tested-by: Kai Huang <kai.huang@intel.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Link: https://patch.msgid.link/20260108214622.1084057-5-michael.roth@amd.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Documentation/virt/kvm/x86/amd-memory-encryption.rst

@@ -523,7 +523,7 @@ Returns: 0 on success, < 0 on error, -EAGAIN if caller should retry
 
         struct kvm_sev_snp_launch_update {
                 __u64 gfn_start;        /* Guest page number to load/encrypt data into. */
-                __u64 uaddr;            /* Userspace address of data to be loaded/encrypted. */
+                __u64 uaddr;            /* 4k-aligned address of data to be loaded/encrypted. */
                 __u64 len;              /* 4k-aligned length in bytes to copy into guest memory.*/
                 __u8 type;              /* The type of the guest pages being initialized. */
                 __u8 pad0;