x86/sev: Add helper for validating pages in early enc attribute changes

early_set_memory_{encrypted,decrypted}() are used for changing the page
state from decrypted (shared) to encrypted (private) and vice versa.

When SEV-SNP is active, the page state transition needs to go through
additional steps.

If the page is transitioned from shared to private, then perform the
following after the encryption attribute is set in the page table:

1. Issue the page state change VMGEXIT to add the page as a private
   in the RMP table.
2. Validate the page after its successfully added in the RMP table.

To maintain the security guarantees, if the page is transitioned from
private to shared, then perform the following before clearing the
encryption attribute from the page table.

1. Invalidate the page.
2. Issue the page state change VMGEXIT to make the page shared in the
   RMP table.

early_set_memory_{encrypted,decrypted}() can be called before the GHCB
is setup so use the SNP page state MSR protocol VMGEXIT defined in the
GHCB specification to request the page state change in the RMP table.

While at it, add a helper snp_prep_memory() which will be used in
probe_roms(), in a later patch.

  [ bp: Massage commit message. ]

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Venu Busireddy <venu.busireddy@oracle.com>
Link: https://lore.kernel.org/r/20220307213356.2797205-19-brijesh.singh@amd.com