[PATCH 2/5] x86/PV: fold redundant calls to adjust_guest_l<N>e()

[Jan Beulich <jbeulich@xxxxxxxx>]

At least from an abstract perspective it is quite odd for us to compare
adjusted old and unadjusted new page table entries when determining
whether the fast path can be used. This is largely benign because
FASTPATH_FLAG_WHITELIST covers most of the flags which the adjustments
may set, and the flags getting set don't affect the outcome of
get_page_from_l<N>e(). There's one exception: 32-bit L3 entries get
_PAGE_RW set, but get_page_from_l3e() doesn't allow linear page tables
to be created at this level for such guests. Apart from this _PAGE_RW
is unused by get_page_from_l<N>e() (for N > 1), and hence forcing the
bit on early has no functional effect.

The main reason for the change, however, is that adjust_guest_l<N>e()
aren't exactly cheap - both in terms of pure code size and because each
one has at least one evaluate_nospec() by way of containing
is_pv_32bit_domain() conditionals.

Call the functions once ahead of the fast path checks, instead of twice
after.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -2199,10 +2199,11 @@ static int mod_l1_entry(l1_pgentry_t *pl
             nl1e = l1e_from_page(page, l1e_get_flags(nl1e));
         }
 
+        nl1e = adjust_guest_l1e(nl1e, pt_dom);
+
         /* Fast path for sufficiently-similar mappings. */
         if ( !l1e_has_changed(ol1e, nl1e, ~FASTPATH_FLAG_WHITELIST) )
         {
-            nl1e = adjust_guest_l1e(nl1e, pt_dom);
             rc = UPDATE_ENTRY(l1, pl1e, ol1e, nl1e, gl1mfn, pt_vcpu,
                               preserve_ad);
             if ( page )
@@ -2227,7 +2228,6 @@ static int mod_l1_entry(l1_pgentry_t *pl
         if ( page )
             put_page(page);
 
-        nl1e = adjust_guest_l1e(nl1e, pt_dom);
         if ( unlikely(!UPDATE_ENTRY(l1, pl1e, ol1e, nl1e, gl1mfn, pt_vcpu,
                                     preserve_ad)) )
         {
@@ -2279,10 +2279,11 @@ static int mod_l2_entry(l2_pgentry_t *pl
             return -EINVAL;
         }
 
+        nl2e = adjust_guest_l2e(nl2e, d);
+
         /* Fast path for sufficiently-similar mappings. */
         if ( !l2e_has_changed(ol2e, nl2e, ~FASTPATH_FLAG_WHITELIST) )
         {
-            nl2e = adjust_guest_l2e(nl2e, d);
             if ( UPDATE_ENTRY(l2, pl2e, ol2e, nl2e, mfn, vcpu, preserve_ad) )
                 return 0;
             return -EBUSY;
@@ -2291,7 +2292,6 @@ static int mod_l2_entry(l2_pgentry_t *pl
         if ( unlikely((rc = get_page_from_l2e(nl2e, mfn, d, 0)) < 0) )
             return rc;
 
-        nl2e = adjust_guest_l2e(nl2e, d);
         if ( unlikely(!UPDATE_ENTRY(l2, pl2e, ol2e, nl2e, mfn, vcpu,
                                     preserve_ad)) )
         {
@@ -2341,10 +2341,11 @@ static int mod_l3_entry(l3_pgentry_t *pl
             return -EINVAL;
         }
 
+        nl3e = adjust_guest_l3e(nl3e, d);
+
         /* Fast path for sufficiently-similar mappings. */
         if ( !l3e_has_changed(ol3e, nl3e, ~FASTPATH_FLAG_WHITELIST) )
         {
-            nl3e = adjust_guest_l3e(nl3e, d);
             rc = UPDATE_ENTRY(l3, pl3e, ol3e, nl3e, mfn, vcpu, preserve_ad);
             return rc ? 0 : -EFAULT;
         }
@@ -2354,7 +2355,6 @@ static int mod_l3_entry(l3_pgentry_t *pl
             return rc;
         rc = 0;
 
-        nl3e = adjust_guest_l3e(nl3e, d);
         if ( unlikely(!UPDATE_ENTRY(l3, pl3e, ol3e, nl3e, mfn, vcpu,
                                     preserve_ad)) )
         {
@@ -2403,10 +2403,11 @@ static int mod_l4_entry(l4_pgentry_t *pl
             return -EINVAL;
         }
 
+        nl4e = adjust_guest_l4e(nl4e, d);
+
         /* Fast path for sufficiently-similar mappings. */
         if ( !l4e_has_changed(ol4e, nl4e, ~FASTPATH_FLAG_WHITELIST) )
         {
-            nl4e = adjust_guest_l4e(nl4e, d);
             rc = UPDATE_ENTRY(l4, pl4e, ol4e, nl4e, mfn, vcpu, preserve_ad);
             return rc ? 0 : -EFAULT;
         }
@@ -2416,7 +2417,6 @@ static int mod_l4_entry(l4_pgentry_t *pl
             return rc;
         rc = 0;
 
-        nl4e = adjust_guest_l4e(nl4e, d);
         if ( unlikely(!UPDATE_ENTRY(l4, pl4e, ol4e, nl4e, mfn, vcpu,
                                     preserve_ad)) )
         {