x86/pv: Inject #GP for implicit grant unmaps

This is a debug behaviour to identify buggy kernels.  Crashing the domain is
the most unhelpful thing to do, because it discards the relevant context.

Instead, inject #GP[0] like other permission errors in x86.  In particular,
this lets the kernel provide a backtrace which is more likely to be helpful to
a developer.

As a bugfix, this always injects #GP[0] to current, not l1e_owner.  It is not
l1e_owner's fault if dom0 using superpowers triggers an implicit unmap.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/mm.c

@@ -1229,10 +1229,10 @@ void put_page_from_l1e(l1_pgentry_t l1e, struct domain *l1e_owner)
     if ( (l1e_get_flags(l1e) & _PAGE_GNTTAB) &&
          !l1e_owner->is_shutting_down && !l1e_owner->is_dying )
     {
-        gdprintk(XENLOG_WARNING,
-                 "Attempt to implicitly unmap a granted PTE %" PRIpte "\n",
-                 l1e_get_intpte(l1e));
-        domain_crash(l1e_owner);
+        gprintk(XENLOG_WARNING,
+                "Attempt to implicitly unmap %pd's grant PTE %" PRIpte "\n",
+                l1e_owner, l1e_get_intpte(l1e));
+        pv_inject_hw_exception(TRAP_gp_fault, 0);
     }
 #endif