CA-422187: safer defaults for global claims

Xen may have already allocated some memory for the domain, and the overhead is
only an estimate.
A global claim failing is a hard failure, so instead use a more conservative
estimate: `memory.build_start_mib`.
This is similar to `required_host_free_mib`, but doesn't take overhead into
account.

Eventually we'd want to have another argument to the create hypercall that
tells it what NUMA node(s) to use, and then we can include all the overhead too
there.

For the single node claim keep the amount as it was, it is only a best effort
claim.

Fixes: 060d792 ("CA-422188: either always use claims or never use claims")

Signed-off-by: Edwin Török <edwin.torok@citrix.com>

Author: edwintorok

ocaml/xenopsd/xc/domain.ml

@@ -1115,6 +1115,8 @@ let build_pre ~xc ~xs ~vcpus ~memory ~hard_affinity domid =
               in
               let memory =
                 Int64.(mul memory.required_host_free_mib (shift_left 1L 20))
+              and memory_hard =
+                Int64.(mul memory.build_start_mib (shift_left 1L 20))
               in
               match numa_placement domid ~vcpus ~cores ~memory affinity with
               | None ->
@@ -1129,7 +1131,7 @@ let build_pre ~xc ~xs ~vcpus ~memory ~hard_affinity domid =
                      A failure here is a hard failure: we'd fail allocating
                      memory later anyway
                   *)
-                  let nr_pages = Int64.div memory 4096L |> Int64.to_int in
+                  let nr_pages = Int64.div memory_hard 4096L |> Int64.to_int in
                   let xcext = Xenctrlext.get_handle () in
                   D.debug "NUMAClaim domid %d: global claim: %d pages" domid
                     nr_pages ;