libxc: use an explicit check for PV MSRs in xc_domain_save()

Migrating PV domains using MSRs is not supported.  This uses the new
XEN_DOMCTL_get_vcpu_msrs and will fail the migration with an explicit error.

This is an improvement upon the current failure of
  "No extended context for VCPUxx (ENOBUFS)"

Support for migrating PV domains which are using MSRs will be included in the
migration v2 work.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>

diff --git a/tools/libxc/xc_domain_save.c b/tools/libxc/xc_domain_save.c
index acf36854f54d8e392eb2cb6272abdb8b7fac4f14..778cbdef39e2f15ad13ef95dcab22ae161f23be6 100644 (file)
--- a/tools/libxc/xc_domain_save.c
+++ b/tools/libxc/xc_domain_save.c
@@ -1995,6 +1995,26 @@ int xc_domain_save(xc_interface *xch, int io_fd, uint32_t dom, uint32_t max_iter
             goto out;
         }
 
+        /* Check there are no PV MSRs in use. */
+        domctl.cmd = XEN_DOMCTL_get_vcpu_msrs;
+        domctl.domain = dom;
+        memset(&domctl.u, 0, sizeof(domctl.u));
+        domctl.u.vcpu_msrs.vcpu = i;
+        domctl.u.vcpu_msrs.msr_count = 0;
+        set_xen_guest_handle_raw(domctl.u.vcpu_msrs.msrs, (void*)1);
+
+        if ( xc_domctl(xch, &domctl) < 0 )
+        {
+            if ( errno == ENOBUFS )
+            {
+                errno = EOPNOTSUPP;
+                PERROR("Unable to migrate PV guest using MSRs (yet)");
+            }
+            else
+                PERROR("Error querying maximum number of MSRs for VCPU%d", i);
+            goto out;
+        }
+
         /* Start to fetch CPU eXtended States */
         /* Get buffer size first */
         domctl.cmd = XEN_DOMCTL_getvcpuextstate;