[PATCH 07/16] x86/shadow: call sh_update_cr3() directly from sh_page_fault()

[Jan Beulich <jbeulich@suse.com>]

There's no need for an indirect call here, as the mode is invariant
throughout the entire paging-locked region. All it takes to avoid it is
to have a forward declaration of sh_update_cr3() in place.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
I find this and the respective Win7 related comment suspicious: If we
really need to "fix up" L3 entries "on demand", wouldn't we better retry
the shadow_get_and_create_l1e() rather than exit? The spurious page
fault that the guest observes can, after all, not be known to be non-
fatal inside the guest. That's purely an OS policy.

Furthermore the sh_update_cr3() will also invalidate L3 entries which
were loaded successfully before, but invalidated by the guest
afterwards. I strongly suspect that the described hardware behavior is
_only_ to load previously not-present entries from the PDPT, but not
purge ones already marked present. IOW I think sh_update_cr3() would
need calling in an "incremental" mode here. (The alternative of doing
this in shadow_get_and_create_l3e() instead would likely be more
cumbersome.)

In any event emitting a TRC_SHADOW_DOMF_DYING trace record in this case
looks wrong.

Beyond the "on demand" L3 entry creation I also can't see what guest
actions could lead to the ASSERT() being inapplicable in the PAE case.
The 3-level code in shadow_get_and_create_l2e() doesn't consult guest
PDPTEs, and all other logic is similar to that for other modes.

(See 89329d832aed ["x86 shadow: Update cr3 in PAE mode when guest walk
succeed but shadow walk fails"].)

--- a/xen/arch/x86/mm/shadow/multi.c
+++ b/xen/arch/x86/mm/shadow/multi.c
@@ -91,6 +91,8 @@ const char *const fetch_type_names[] = {
 # define for_each_shadow_table(v, i) for ( (i) = 0; (i) < 1; ++(i) )
 #endif
 
+static void cf_check sh_update_cr3(struct vcpu *v, int do_locking, bool noflush);
+
 /* Helper to perform a local TLB flush. */
 static void sh_flush_local(const struct domain *d)
 {
@@ -2487,7 +2489,7 @@ static int cf_check sh_page_fault(
          * In any case, in the PAE case, the ASSERT is not true; it can
          * happen because of actions the guest is taking. */
 #if GUEST_PAGING_LEVELS == 3
-        v->arch.paging.mode->update_cr3(v, 0, false);
+        sh_update_cr3(v, 0, false);
 #else
         ASSERT(d->is_shutting_down);
 #endif