[PATCH v2 19/22] KVM: MMU: lockless walking shadow page table

Xiao Guangrong
2011-06-22T14:35:32Z

Use rcu to protect shadow pages table to be freed, so we can safely walk it,
it should run fastly and is needed by mmio page fault

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
---
arch/x86/include/asm/kvm_host.h | 8 +++
arch/x86/kvm/mmu.c | 132 ++++++++++++++++++++++++++++++++++++---
2 files changed, 132 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 42e577d..87a868e 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -233,6 +233,12 @@ struct kvm_mmu_page {
unsigned int unsync_children;
unsigned long parent_ptes; /* Reverse mapping for parent_pte */
DECLARE_BITMAP(unsync_child_bitmap, 512);
+
+#ifdef CONFIG_X86_32
+ int clear_spte_count;
+#endif
+
+ struct rcu_head rcu;
};

struct kvm_pv_mmu_op_buffer {
@@ -477,6 +483,8 @@ struct kvm_arch {
u64 hv_guest_os_id;
u64 hv_hypercall;

+ atomic_t reader_counter;
+
#ifdef CONFIG_KVM_MMU_AUDIT
int audit_point;
#endif
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 0ba94e9..dad7ad9 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -182,6 +182,12 @@ struct kvm_shadow_walk_iterator {
shadow_walk_okay(&(_walker)); \
shadow_walk_next(&(_walker)))

+#define for_each_shadow_entry_lockless(_vcpu, _addr, _walker, spte) \
+ for (shadow_walk_init(&(_walker), _vcpu, _addr); \
+ shadow_walk_okay(&(_walker)) && \
+ ({ spte = mmu_spte_get_lockless(_walker.sptep); 1; }); \
+ __shadow_walk_next(&(_walker), spte))
+
static struct kmem_cache *pte_list_desc_cache;
static struct kmem_cache *mmu_page_header_cache;
static struct percpu_counter kvm_total_used_mmu_pages;
@@ -274,6 +280,11 @@ static u64 __update_clear_spte_slow(u64 *sptep, u64 spte)
{
return xchg(sptep, spte);
}
+
+static u64 __get_spte_lockless(u64 *sptep)
+{
+ return ACCESS_ONCE(*sptep);
+}
#else
union split_spte {
struct {
@@ -283,6 +294,18 @@ union split_spte {
u64 spte;
};

+static void count_spte_clear(u64 *sptep, u64 spte)
+{
+ struct kvm_mmu_page *sp = page_header(__pa(sptep));
+
+ if (is_shadow_present_pte(spte))
+ return;
+
+ /* Ensure the spte is completely set before we increase the count */
+ smp_wmb();
+ sp->clear_spte_count++;
+}
+
static void __set_spte(u64 *sptep, u64 spte)
{
union split_spte *ssptep, sspte;
@@ -318,6 +341,7 @@ static void __update_clear_spte_fast(u64 *sptep, u64 spte)
smp_wmb();

ssptep->spte_high = sspte.spte_high;
+ count_spte_clear(sptep, spte);
}

static u64 __update_clear_spte_slow(u64 *sptep, u64 spte)
@@ -330,9 +354,40 @@ static u64 __update_clear_spte_slow(u64 *sptep, u64 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;
+ count_spte_clear(sptep, spte);

return orig.spte;
}
+
+/*
+ * The idea using the light way get the spte on x86_32 guest is from
+ * gup_get_pte(arch/x86/mm/gup.c).
+ * The difference is we can not catch the spte tlb flush if we leave
+ * guest mode, so we emulate it by increase clear_spte_count when spte
+ * is cleared.
+ */
+static u64 __get_spte_lockless(u64 *sptep)
+{
+ struct kvm_mmu_page *sp = page_header(__pa(sptep));
+ union split_spte spte, *orig = (union split_spte *)sptep;
+ int count;
+
+retry:
+ count = sp->clear_spte_count;
+ smp_rmb();
+
+ spte.spte_low = orig->spte_low;
+ smp_rmb();
+
+ spte.spte_high = orig->spte_high;
+ smp_rmb();
+
+ if (unlikely(spte.spte_low != orig->spte_low ||
+ count != sp->clear_spte_count))
+ goto retry;
+
+ return spte.spte;
+}
#endif

static bool spte_is_bit_cleared(u64 old_spte, u64 new_spte, u64 bit_mask)
@@ -438,6 +493,28 @@ static void mmu_spte_clear_no_track(u64 *sptep)
__update_clear_spte_fast(sptep, 0ull);
}

+static u64 mmu_spte_get_lockless(u64 *sptep)
+{
+ return __get_spte_lockless(sptep);
+}
+
+static void walk_shadow_page_lockless_begin(struct kvm_vcpu *vcpu)
+{
+ rcu_read_lock();
+ atomic_inc(&vcpu->kvm->arch.reader_counter);
+
+ /* Increase the counter before walking shadow page table */
+ smp_mb__after_atomic_inc();
+}
+
+static void walk_shadow_page_lockless_end(struct kvm_vcpu *vcpu)
+{
+ /* Decrease the counter after walking shadow page table finished */
+ smp_mb__before_atomic_dec();
+ atomic_dec(&vcpu->kvm->arch.reader_counter);
+ rcu_read_unlock();
+}
+
static int mmu_topup_memory_cache(struct kvm_mmu_memory_cache *cache,
struct kmem_cache *base_cache, int min)
{
@@ -1600,17 +1677,23 @@ static bool shadow_walk_okay(struct kvm_shadow_walk_iterator *iterator)
return true;
}

-static void shadow_walk_next(struct kvm_shadow_walk_iterator *iterator)
+static void __shadow_walk_next(struct kvm_shadow_walk_iterator *iterator,
+ u64 spte)
{
- if (is_last_spte(*iterator->sptep, iterator->level)) {
+ if (is_last_spte(spte, iterator->level)) {
iterator->level = 0;
return;
}

- iterator->shadow_addr = *iterator->sptep & PT64_BASE_ADDR_MASK;
+ iterator->shadow_addr = spte & PT64_BASE_ADDR_MASK;
--iterator->level;
}

+static void shadow_walk_next(struct kvm_shadow_walk_iterator *iterator)
+{
+ return __shadow_walk_next(iterator, *iterator->sptep);
+}
+
static void link_shadow_page(u64 *sptep, struct kvm_mmu_page *sp)
{
u64 spte;
@@ -1757,6 +1840,30 @@ static int kvm_mmu_prepare_zap_page(struct kvm *kvm, struct kvm_mmu_page *sp,
return ret;
}

+static void kvm_mmu_isolate_pages(struct list_head *invalid_list)
+{
+ struct kvm_mmu_page *sp;
+
+ list_for_each_entry(sp, invalid_list, link)
+ kvm_mmu_isolate_page(sp);
+}
+
+static void free_pages_rcu(struct rcu_head *head)
+{
+ struct kvm_mmu_page *next, *sp;
+
+ sp = container_of(head, struct kvm_mmu_page, rcu);
+ while (sp) {
+ if (!list_empty(&sp->link))
+ next = list_first_entry(&sp->link,
+ struct kvm_mmu_page, link);
+ else
+ next = NULL;
+ kvm_mmu_free_page(sp);
+ sp = next;
+ }
+}
+
static void kvm_mmu_commit_zap_page(struct kvm *kvm,
struct list_head *invalid_list)
{
@@ -1767,6 +1874,14 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,

kvm_flush_remote_tlbs(kvm);

+ if (atomic_read(&kvm->arch.reader_counter)) {
+ kvm_mmu_isolate_pages(invalid_list);
+ sp = list_first_entry(invalid_list, struct kvm_mmu_page, link);
+ list_del_init(invalid_list);
+ call_rcu(&sp->rcu, free_pages_rcu);
+ return;
+ }
+
do {
sp = list_first_entry(invalid_list, struct kvm_mmu_page, link);
WARN_ON(!sp->role.invalid || sp->root_count);
@@ -3797,16 +3912,17 @@ out:
int kvm_mmu_get_spte_hierarchy(struct kvm_vcpu *vcpu, u64 addr, u64 sptes[4])
{
struct kvm_shadow_walk_iterator iterator;
+ u64 spte;
int nr_sptes = 0;

- spin_lock(&vcpu->kvm->mmu_lock);
- for_each_shadow_entry(vcpu, addr, iterator) {
- sptes[iterator.level-1] = *iterator.sptep;
+ walk_shadow_page_lockless_begin(vcpu);
+ for_each_shadow_entry_lockless(vcpu, addr, iterator, spte) {
+ sptes[iterator.level-1] = spte;
nr_sptes++;
- if (!is_shadow_present_pte(*iterator.sptep))
+ if (!is_shadow_present_pte(spte))
break;
}
- spin_unlock(&vcpu->kvm->mmu_lock);
+ walk_shadow_page_lockless_end(vcpu);

return nr_sptes;
}
--
1.7.5.4