[Xen-devel] [RFC PATCH] p2m-pt: avoid get the pte falgs repeatedly.

[Liang Li <liang.z.li@xxxxxxxxx>]

There are a lot of code try to get the pte flags repeatedly, why
not save the result and reuse it in the following code? It can help
to save some CPU cycles and make the code cleaner, no?

I am not sure if this is the right direction, just change one place.

Signed-off-by: Liang Li <liang.z.li@xxxxxxxxx>
---
 xen/arch/x86/mm/p2m-pt.c | 35 +++++++++++++++++++++--------------
 1 file changed, 21 insertions(+), 14 deletions(-)

diff --git a/xen/arch/x86/mm/p2m-pt.c b/xen/arch/x86/mm/p2m-pt.c
index 3b025d5..bbaa54f 100644
--- a/xen/arch/x86/mm/p2m-pt.c
+++ b/xen/arch/x86/mm/p2m-pt.c
@@ -64,7 +64,7 @@
 #define clear_recalc(level, ent) level##e_add_flags(ent, RECALC_FLAGS)
 #define _needs_recalc(flags) (!((flags) & _PAGE_USER))
 #define needs_recalc(level, ent) _needs_recalc(level##e_get_flags(ent))
-#define valid_recalc(level, ent) (!(level##e_get_flags(ent) & _PAGE_ACCESSED))
+#define valid_recalc(flags) (!(flags & _PAGE_ACCESSED))
 
 static const unsigned long pgt[] = {
     PGT_l1_page_table,
@@ -367,7 +367,7 @@ static int do_recalc(struct p2m_domain *p2m, unsigned long 
gfn)
 {
     void *table;
     unsigned long gfn_remainder = gfn;
-    unsigned int level = 4;
+    unsigned int level = 4, pte_flags;
     l1_pgentry_t *pent;
     int err = 0;
 
@@ -378,15 +378,20 @@ static int do_recalc(struct p2m_domain *p2m, unsigned 
long gfn)
 
         pent = p2m_find_entry(table, &remainder, gfn,
                               level * PAGETABLE_ORDER, 1 << PAGETABLE_ORDER);
-        if ( !pent || !(l1e_get_flags(*pent) & _PAGE_PRESENT) )
+        if ( !pent )
+            goto out;
+       else
+            pte_flags = l1e_get_flags(*pent);
+
+        if ( !(pte_flags & _PAGE_PRESENT) )
             goto out;
 
-        if ( l1e_get_flags(*pent) & _PAGE_PSE )
+        if ( pte_flags & _PAGE_PSE )
         {
             unsigned long mask = ~0UL << (level * PAGETABLE_ORDER);
 
-            if ( !needs_recalc(l1, *pent) ||
-                 !p2m_is_changeable(p2m_flags_to_type(l1e_get_flags(*pent))) ||
+            if ( !_needs_recalc(pte_flags) ||
+                 !p2m_is_changeable(p2m_flags_to_type(pte_flags)) ||
                  p2m_is_logdirty_range(p2m, gfn & mask, gfn | ~mask) >= 0 )
                 break;
         }
@@ -397,21 +402,21 @@ static int do_recalc(struct p2m_domain *p2m, unsigned 
long gfn)
         if ( err )
             goto out;
 
-        if ( needs_recalc(l1, *pent) )
+        if ( _needs_recalc(pte_flags) )
         {
             l1_pgentry_t e = *pent, *ptab = table;
             unsigned int i;
 
-            if ( !valid_recalc(l1, e) )
+            if ( !valid_recalc(pte_flags) )
                 P2M_DEBUG("bogus recalc state at d%d:%lx:%u\n",
                           p2m->domain->domain_id, gfn, level);
             remainder = gfn_remainder;
             for ( i = 0; i < (1 << PAGETABLE_ORDER); ++i )
             {
                 l1_pgentry_t ent = ptab[i];
+                int flags = l1e_get_flags(ent);
 
-                if ( (l1e_get_flags(ent) & _PAGE_PRESENT) &&
-                     !needs_recalc(l1, ent) )
+                if ( (flags & _PAGE_PRESENT) && !_needs_recalc(flags) )
                 {
                     set_recalc(l1, ent);
                     p2m->write_p2m_entry(p2m, gfn - remainder, &ptab[i],
@@ -428,15 +433,17 @@ static int do_recalc(struct p2m_domain *p2m, unsigned 
long gfn)
 
     pent = p2m_find_entry(table, &gfn_remainder, gfn,
                           level * PAGETABLE_ORDER, 1 << PAGETABLE_ORDER);
-    if ( pent && (l1e_get_flags(*pent) & _PAGE_PRESENT) &&
-         needs_recalc(l1, *pent) )
+    if (!pent)
+        goto out;
+    pte_flags = l1e_get_flags(*pent);
+    if ( (pte_flags & _PAGE_PRESENT) && _needs_recalc(pte_flags) )
     {
         l1_pgentry_t e = *pent;
 
-        if ( !valid_recalc(l1, e) )
+        if ( !valid_recalc(pte_flags) )
             P2M_DEBUG("bogus recalc leaf at d%d:%lx:%u\n",
                       p2m->domain->domain_id, gfn, level);
-        if ( p2m_is_changeable(p2m_flags_to_type(l1e_get_flags(e))) )
+        if ( p2m_is_changeable(p2m_flags_to_type(pte_flags)) )
         {
             unsigned long mask = ~0UL << (level * PAGETABLE_ORDER);
             p2m_type_t p2mt = p2m_is_logdirty_range(p2m, gfn & mask, gfn | 
~mask)
-- 
1.9.1


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