[PATCH][4.17] x86/shadow: drop (replace) bogus assertions

[Jan Beulich <jbeulich@xxxxxxxx>]

The addition of a call to shadow_blow_tables() from shadow_teardown()
has resulted in the "no vcpus" related assertion becoming triggerable:
If domain_create() fails with at least one page successfully allocated
in the course of shadow_enable(), or if domain_create() succeeds and
the domain is then killed without ever invoking XEN_DOMCTL_max_vcpus.

The assertion's comment was bogus anyway: Shadow mode has been getting
enabled before allocation of vCPU-s for quite some time. Convert the
assertion to a conditional: As long as there are no vCPU-s, there's
nothing to blow away.

Fixes: e7aa55c0aab3 ("x86/p2m: free the paging memory pool preemptively")
Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

A similar assertion/comment pair exists in _shadow_prealloc(); the
comment is similarly bogus, and the assertion could in principle trigger
e.g. when shadow_alloc_p2m_page() is called early enough. Replace those
at the same time by a similar early return, here indicating failure to
the caller (which will generally lead to the domain being crashed in
shadow_prealloc()).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
While in shadow_blow_tables() the option exists to simply remove the
assertion without adding a new conditional (the two loops simply will
do nothing), the same isn't true for _shadow_prealloc(): There we
would then trigger the ASSERT_UNREACHABLE() near the end of the
function.

--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -943,8 +943,9 @@ static bool __must_check _shadow_preallo
         /* No reclaim when the domain is dying, teardown will take care of it. 
*/
         return false;
 
-    /* Shouldn't have enabled shadows if we've no vcpus. */
-    ASSERT(d->vcpu && d->vcpu[0]);
+    /* Nothing to reclaim when there are no vcpus yet. */
+    if ( !d->vcpu[0] )
+        return false;
 
     /* Stage one: walk the list of pinned pages, unpinning them */
     perfc_incr(shadow_prealloc_1);
@@ -1034,8 +1035,9 @@ void shadow_blow_tables(struct domain *d
     mfn_t smfn;
     int i;
 
-    /* Shouldn't have enabled shadows if we've no vcpus. */
-    ASSERT(d->vcpu && d->vcpu[0]);
+    /* Nothing to do when there are no vcpus yet. */
+    if ( !d->vcpu[0] )
+        return;
 
     /* Pass one: unpin all pinned pages */
     foreach_pinned_shadow(d, sp, t)