[PATCH v3 08/15] KVM: MMU: introduce pte-list lockless walker

Xiao Guangrong
2013-10-23T13:33:21Z

The basic idea is from nulls list which uses a nulls to indicate
whether the desc is moved to different pte-list

Note, we should do bottom-up walk in the desc since we always move
the bottom entry to the deleted position

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 | 57 ++++++++++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 53 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 4687329..a864140 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -975,6 +975,10 @@ static int count_spte_number(struct pte_list_desc *desc)
return first_free + desc_num * PTE_LIST_EXT;
}

+#define rcu_assign_pte_list(pte_list_p, value) \
+ rcu_assign_pointer(*(unsigned long __rcu **)(pte_list_p), \
+ (unsigned long *)(value))
+
/*
* Pte mapping structures:
*
@@ -994,7 +998,7 @@ static int pte_list_add(struct kvm_vcpu *vcpu, u64 *spte,

if (!*pte_list) {
rmap_printk("pte_list_add: %p %llx 0->1\n", spte, *spte);
- *pte_list = (unsigned long)spte;
+ rcu_assign_pte_list(pte_list, spte);
return 0;
}

@@ -1004,7 +1008,7 @@ 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);
- *pte_list = (unsigned long)desc | 1;
+ rcu_assign_pte_list(pte_list, (unsigned long)desc | 1);
return 1;
}

@@ -1017,7 +1021,7 @@ static int pte_list_add(struct kvm_vcpu *vcpu, u64 *spte,
new_desc = mmu_alloc_pte_list_desc(vcpu);
new_desc->more = desc;
desc = new_desc;
- *pte_list = (unsigned long)desc | 1;
+ rcu_assign_pte_list(pte_list, (unsigned long)desc | 1);
}

free_pos = find_first_free(desc);
@@ -1125,6 +1129,51 @@ 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. */
+static void pte_list_walk_lockless(unsigned long *pte_list,
+ pte_list_walk_fn fn)
+{
+ struct pte_list_desc *desc;
+ unsigned long pte_list_value;
+ int i;
+
+restart:
+ /*
+ * Force the pte_list to be reloaded.
+ *
+ * See the comments in hlist_nulls_for_each_entry_rcu().
+ */
+ barrier();
+ pte_list_value = *rcu_dereference(pte_list);
+ if (!pte_list_value)
+ return;
+
+ if (!(pte_list_value & 1))
+ return fn((u64 *)pte_list_value);
+
+ desc = (struct pte_list_desc *)(pte_list_value & ~1ul);
+ while (!desc_is_a_nulls(desc)) {
+ /*
+ * We should do top-down walk since we always use the higher
+ * indices to replace the deleted entry if only one desc is
+ * used in the rmap when a spte is removed. Otherwise the
+ * moved entry will be missed.
+ */
+ for (i = PTE_LIST_EXT - 1; i >= 0; i--)
+ if (desc->sptes[i])
+ fn(desc->sptes[i]);
+
+ desc = rcu_dereference(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)
{
@@ -4615,7 +4664,7 @@ 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