[xen staging] x86/PV: fold redundant calls to adjust_guest_l<N>e()

[patchbot@xxxxxxx]

commit 2039ebfd59eef01176f5d26e83654f8bfbe0a949
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon Jan 11 14:51:39 2021 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Jan 11 14:51:39 2021 +0100

    x86/PV: fold redundant calls to adjust_guest_l<N>e()
    
    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>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/mm.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 79acf20c4e..63e9fae919 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -2195,10 +2195,11 @@ static int mod_l1_entry(l1_pgentry_t *pl1e, 
l1_pgentry_t nl1e,
             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 )
@@ -2223,7 +2224,6 @@ static int mod_l1_entry(l1_pgentry_t *pl1e, l1_pgentry_t 
nl1e,
         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)) )
         {
@@ -2275,10 +2275,11 @@ static int mod_l2_entry(l2_pgentry_t *pl2e,
             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;
@@ -2287,7 +2288,6 @@ static int mod_l2_entry(l2_pgentry_t *pl2e,
         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)) )
         {
@@ -2337,10 +2337,11 @@ static int mod_l3_entry(l3_pgentry_t *pl3e,
             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;
         }
@@ -2350,7 +2351,6 @@ static int mod_l3_entry(l3_pgentry_t *pl3e,
             return rc;
         rc = 0;
 
-        nl3e = adjust_guest_l3e(nl3e, d);
         if ( unlikely(!UPDATE_ENTRY(l3, pl3e, ol3e, nl3e, mfn, vcpu,
                                     preserve_ad)) )
         {
@@ -2399,10 +2399,11 @@ static int mod_l4_entry(l4_pgentry_t *pl4e,
             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;
         }
@@ -2412,7 +2413,6 @@ static int mod_l4_entry(l4_pgentry_t *pl4e,
             return rc;
         rc = 0;
 
-        nl4e = adjust_guest_l4e(nl4e, d);
         if ( unlikely(!UPDATE_ENTRY(l4, pl4e, ol4e, nl4e, mfn, vcpu,
                                     preserve_ad)) )
         {
--
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