KVM: MMU: do not need atomicly to set/clear spte

Now, the spte is just from nonprsent to present or present to nonprsent, so
we can use some trick to set/clear spte non-atomicly as linux kernel does

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 71 insertions, 15 deletions

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 982718fe12a7..a22b5fe92167 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -259,26 +259,82 @@ static gfn_t pse36_gfn_delta(u32 gpte)
         return (gpte & PT32_DIR_PSE36_MASK) << shift;
 }
 
+#ifdef CONFIG_X86_64
 static void __set_spte(u64 *sptep, u64 spte)
 {
-        set_64bit(sptep, spte);
+        *sptep = spte;
 }
 
-static u64 __xchg_spte(u64 *sptep, u64 new_spte)
+static void __update_clear_spte_fast(u64 *sptep, u64 spte)
 {
-#ifdef CONFIG_X86_64
-        return xchg(sptep, new_spte);
+        *sptep = spte;
+}
+
+static u64 __update_clear_spte_slow(u64 *sptep, u64 spte)
+{
+        return xchg(sptep, spte);
+}
 #else
-        u64 old_spte;
+union split_spte {
+        struct {
+                u32 spte_low;
+                u32 spte_high;
+        };
+        u64 spte;
+};
 
-        do {
-                old_spte = *sptep;
-        } while (cmpxchg64(sptep, old_spte, new_spte) != old_spte);
+static void __set_spte(u64 *sptep, u64 spte)
+{
+        union split_spte *ssptep, sspte;
 
-        return old_spte;
-#endif
+        ssptep = (union split_spte *)sptep;
+        sspte = (union split_spte)spte;
+
+        ssptep->spte_high = sspte.spte_high;
+
+        /*
+         * If we map the spte from nonpresent to present, We should store
+         * the high bits firstly, then set present bit, so cpu can not
+         * fetch this spte while we are setting the spte.
+         */
+        smp_wmb();
+
+        ssptep->spte_low = sspte.spte_low;
 }
 
+static void __update_clear_spte_fast(u64 *sptep, u64 spte)
+{
+        union split_spte *ssptep, sspte;
+
+        ssptep = (union split_spte *)sptep;
+        sspte = (union split_spte)spte;
+
+        ssptep->spte_low = sspte.spte_low;
+
+        /*
+         * If we map the spte from present to nonpresent, we should clear
+         * present bit firstly to avoid vcpu fetch the old high bits.
+         */
+        smp_wmb();
+
+        ssptep->spte_high = sspte.spte_high;
+}
+
+static u64 __update_clear_spte_slow(u64 *sptep, u64 spte)
+{
+        union split_spte *ssptep, sspte, orig;
+
+        ssptep = (union split_spte *)sptep;
+        sspte = (union split_spte)spte;
+
+        /* xchg acts as a barrier before the setting of the high bits */
+        orig.spte_low = xchg(&ssptep->spte_low, sspte.spte_low);
+        orig.spte_high = ssptep->spte_high = sspte.spte_high;
+
+        return orig.spte;
+}
+#endif
+
 static bool spte_has_volatile_bits(u64 spte)
 {
         if (!shadow_accessed_mask)
@@ -330,9 +386,9 @@ static void mmu_spte_update(u64 *sptep, u64 new_spte)
                 mask |= shadow_dirty_mask;
 
         if (!spte_has_volatile_bits(old_spte) || (new_spte & mask) == mask)
-                __set_spte(sptep, new_spte);
+                __update_clear_spte_fast(sptep, new_spte);
         else
-                old_spte = __xchg_spte(sptep, new_spte);
+                old_spte = __update_clear_spte_slow(sptep, new_spte);
 
         if (!shadow_accessed_mask)
                 return;
@@ -354,9 +410,9 @@ static int mmu_spte_clear_track_bits(u64 *sptep)
         u64 old_spte = *sptep;
 
         if (!spte_has_volatile_bits(old_spte))
-                __set_spte(sptep, 0ull);
+                __update_clear_spte_fast(sptep, 0ull);
         else
-                old_spte = __xchg_spte(sptep, 0ull);
+                old_spte = __update_clear_spte_slow(sptep, 0ull);
 
         if (!is_rmap_spte(old_spte))
                 return 0;
@@ -376,7 +432,7 @@ static int mmu_spte_clear_track_bits(u64 *sptep)
  */
 static void mmu_spte_clear_no_track(u64 *sptep)
 {
-        __set_spte(sptep, 0ull);
+        __update_clear_spte_fast(sptep, 0ull);
 }
 
 static int mmu_topup_memory_cache(struct kvm_mmu_memory_cache *cache,