[xen staging] docs: Warn about incomplete vtpmmgr TPM 2.0 support

[patchbot@xxxxxxx]

commit 93b2558fae83ab3a6a9b48c851d48ccf57be2298
Author:     Jason Andryuk <jandryuk@xxxxxxxxx>
AuthorDate: Thu May 6 09:59:11 2021 -0400
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri May 7 19:50:50 2021 +0100

    docs: Warn about incomplete vtpmmgr TPM 2.0 support
    
    The vtpmmgr TPM 2.0 support is incomplete.  Add a warning about that to
    the documentation so others don't have to work through discovering it is
    broken.
    
    Signed-off-by: Jason Andryuk <jandryuk@xxxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Daniel P. Smith <dpsmith@xxxxxxxxxxxxxxxxxxxx>
---
 docs/man/xen-vtpmmgr.7.pod | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/docs/man/xen-vtpmmgr.7.pod b/docs/man/xen-vtpmmgr.7.pod
index af825a7ffe..875dcce508 100644
--- a/docs/man/xen-vtpmmgr.7.pod
+++ b/docs/man/xen-vtpmmgr.7.pod
@@ -222,6 +222,17 @@ XSM label, not the kernel.
 
 =head1 Appendix B: vtpmmgr on TPM 2.0
 
+=head2 WARNING: Incomplete - cannot persist data
+
+TPM 2.0 support for vTPM manager is incomplete.  There is no support for
+persisting an encryption key, so vTPM manager regenerates primary and secondary
+key handles each boot.
+
+Also, the vTPM manger group command implementation hardcodes TPM 1.2 commands.
+This means running manage-vtpmmgr.pl fails when the TPM 2.0 hardware rejects
+the TPM 1.2 commands.  vTPM manager with TPM 2.0 cannot create groups and
+therefore cannot persist vTPM contents.
+
 =head2 Manager disk image setup:
 
 The vTPM Manager requires a disk image to store its encrypted data. The image
--
generated by git-patchbot for /home/xen/git/xen.git#staging