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Subject[PATCH 09/12] KVM: MMU: introduce pte-list lockless walker
Date
The basic idea is from nulls list which uses a nulls to indicate
whether the desc is moved to different pte-list

Thanks to SLAB_DESTROY_BY_RCU, the desc can be quickly reused

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
---
arch/x86/kvm/mmu.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 50 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 36caf6a..f8fc0cc 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1010,6 +1010,14 @@ static int pte_list_add(struct kvm_vcpu *vcpu, u64 *spte,
desc->sptes[0] = (u64 *)*pte_list;
desc->sptes[1] = spte;
desc_mark_nulls(pte_list, desc);
+
+ /*
+ * Esure the old spte has been updated into desc, so
+ * that the another side can not get the desc from pte_list
+ * but miss the old spte.
+ */
+ smp_wmb();
+
*pte_list = (unsigned long)desc | 1;
return 1;
}
@@ -1131,6 +1139,47 @@ static void pte_list_walk(unsigned long *pte_list, pte_list_walk_fn fn)
WARN_ON(desc_get_nulls_value(desc) != pte_list);
}

+/* The caller should hold rcu lock. */
+typedef void (*pte_list_walk_lockless_fn) (u64 *spte, int level);
+static void pte_list_walk_lockless(unsigned long *pte_list,
+ pte_list_walk_lockless_fn fn, int level)
+{
+ struct pte_list_desc *desc;
+ unsigned long pte_list_value;
+ int i;
+
+restart:
+ pte_list_value = ACCESS_ONCE(*pte_list);
+ if (!pte_list_value)
+ return;
+
+ if (!(pte_list_value & 1))
+ return fn((u64 *)pte_list_value, level);
+
+ /*
+ * fetch pte_list before read sptes in the desc, see the comments
+ * in pte_list_add().
+ *
+ * There is the data dependence since the desc is got from pte_list.
+ */
+ smp_read_barrier_depends();
+
+ desc = (struct pte_list_desc *)(pte_list_value & ~1ul);
+ while (!desc_is_a_nulls(desc)) {
+ for (i = 0; i < PTE_LIST_EXT && desc->sptes[i]; ++i)
+ fn(desc->sptes[i], level);
+
+ desc = ACCESS_ONCE(desc->more);
+
+ /* It is being initialized. */
+ if (unlikely(!desc))
+ goto restart;
+ }
+
+ if (unlikely(desc_get_nulls_value(desc) != pte_list))
+ goto restart;
+}
+
static unsigned long *__gfn_to_rmap(gfn_t gfn, int level,
struct kvm_memory_slot *slot)
{
@@ -4557,7 +4606,7 @@ int kvm_mmu_module_init(void)
{
pte_list_desc_cache = kmem_cache_create("pte_list_desc",
sizeof(struct pte_list_desc),
- 0, 0, NULL);
+ 0, SLAB_DESTROY_BY_RCU, NULL);
if (!pte_list_desc_cache)
goto nomem;

--
1.8.1.4


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