[PATCH 18/22] KVM: MMU: do not need atomicly to set/clear spte

Xiao Guangrong
2011-06-22T14:35:11Z

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>
---
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 339e1a3..0ba94e9 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