[PATCH v3 15/19] KVM: MMU: do not need atomicly to set/clear spte

[Xiao Guangrong]

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 <xiaoguangr...@cn.fujitsu.com>
---
 arch/x86/kvm/mmu.c |   82 +++++++++++++++++++++++++++++++++++++++++++--------
 1 files changed, 69 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 857d0d6..06c3a4a 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_is_bit_cleared(u64 old_spte, u64 new_spte, u64 bit_mask)
 {
        return (old_spte & bit_mask) && !(new_spte & bit_mask);
@@ -319,9 +375,9 @@ static u64 spte_get_and_update_clear(u64 *sptep, u64 
new_spte)
        u64 old_spte = *sptep;
 
        if (!spte_has_volatile_bits(old_spte, new_spte))
-               __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);
 
        return old_spte;
 }
@@ -379,7 +435,7 @@ 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,
-- 
1.7.5.4

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