[PATCH v3 13/15] KVM: MMU: locklessly write-protect the page

Xiao Guangrong
2013-10-23T13:30:48Z

Currently, when mark memslot dirty logged or get dirty page, we need to
write-protect large guest memory, it is the heavy work, especially, we
need to hold mmu-lock which is also required by vcpu to fix its page table
fault and mmu-notifier when host page is being changed. In the extreme
cpu / memory used guest, it becomes a scalability issue

This patch introduces a way to locklessly write-protect guest memory

Now, lockless rmap walk, lockless shadow page table access and lockless
spte wirte-protection are ready, it is the time to implements page
write-protection out of mmu-lock

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
---
arch/x86/include/asm/kvm_host.h | 4 ---
arch/x86/kvm/mmu.c | 59 ++++++++++++++++++++++++++++++-----------
arch/x86/kvm/mmu.h | 6 +++++
arch/x86/kvm/x86.c | 11 ++++----
4 files changed, 55 insertions(+), 25 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index df9ae10..cdb6f29 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -793,10 +793,6 @@ void kvm_mmu_set_mask_ptes(u64 user_mask, u64 accessed_mask,
u64 dirty_mask, u64 nx_mask, u64 x_mask);

void kvm_mmu_reset_context(struct kvm_vcpu *vcpu);
-void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot);
-void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
- struct kvm_memory_slot *slot,
- gfn_t gfn_offset, unsigned long mask);
void kvm_mmu_zap_all(struct kvm *kvm);
void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm);
unsigned int kvm_mmu_calculate_mmu_pages(struct kvm *kvm);
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 8b96d96..d82bbec 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1386,8 +1386,37 @@ static bool __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp,
return flush;
}

-/**
- * kvm_mmu_write_protect_pt_masked - write protect selected PT level pages
+static void __rmap_write_protect_lockless(u64 *sptep)
+{
+ u64 spte;
+
+retry:
+ /*
+ * Note we may partly read the sptep on 32bit host, however, we
+ * allow this case because:
+ * - we do not access the page got from the sptep.
+ * - cmpxchg64 can detect that case and avoid setting a wrong value
+ * to the sptep.
+ */
+ spte = *rcu_dereference(sptep);
+ if (unlikely(!is_last_spte(spte) || !is_writable_pte(spte)))
+ return;
+
+ if (likely(cmpxchg64(sptep, spte, spte & ~PT_WRITABLE_MASK) == spte))
+ return;
+
+ goto retry;
+}
+
+static void rmap_write_protect_lockless(unsigned long *rmapp)
+{
+ pte_list_walk_lockless(rmapp, __rmap_write_protect_lockless);
+}
+
+/*
+ * kvm_mmu_write_protect_pt_masked_lockless - write protect selected PT level
+ * pages out of mmu-lock.
+ *
* @kvm: kvm instance
* @slot: slot to protect
* @gfn_offset: start of the BITS_PER_LONG pages we care about
@@ -1396,16 +1425,17 @@ static bool __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp,
* Used when we do not need to care about huge page mappings: e.g. during dirty
* logging we do not have any such mappings.
*/
-void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
- struct kvm_memory_slot *slot,
- gfn_t gfn_offset, unsigned long mask)
+void
+kvm_mmu_write_protect_pt_masked_lockless(struct kvm *kvm,
+ struct kvm_memory_slot *slot,
+ gfn_t gfn_offset, unsigned long mask)
{
unsigned long *rmapp;

while (mask) {
rmapp = __gfn_to_rmap(slot->base_gfn + gfn_offset + __ffs(mask),
PT_PAGE_TABLE_LEVEL, slot);
- __rmap_write_protect(kvm, rmapp, false);
+ rmap_write_protect_lockless(rmapp);

/* clear the first set bit */
mask &= mask - 1;
@@ -4477,7 +4507,7 @@ void kvm_mmu_setup(struct kvm_vcpu *vcpu)
init_kvm_mmu(vcpu);
}

-void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
+void kvm_mmu_slot_remove_write_access_lockless(struct kvm *kvm, int slot)
{
struct kvm_memory_slot *memslot;
gfn_t last_gfn;
@@ -4486,8 +4516,7 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
memslot = id_to_memslot(kvm->memslots, slot);
last_gfn = memslot->base_gfn + memslot->npages - 1;

- spin_lock(&kvm->mmu_lock);
-
+ rcu_read_lock();
for (i = PT_PAGE_TABLE_LEVEL;
i < PT_PAGE_TABLE_LEVEL + KVM_NR_PAGE_SIZES; ++i) {
unsigned long *rmapp;
@@ -4497,15 +4526,15 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
last_index = gfn_to_index(last_gfn, memslot->base_gfn, i);

for (index = 0; index <= last_index; ++index, ++rmapp) {
- if (*rmapp)
- __rmap_write_protect(kvm, rmapp, false);
+ rmap_write_protect_lockless(rmapp);

- if (need_resched() || spin_needbreak(&kvm->mmu_lock))
- cond_resched_lock(&kvm->mmu_lock);
+ if (need_resched()) {
+ rcu_read_lock();
+ rcu_read_unlock();
+ }
}
}
-
- spin_unlock(&kvm->mmu_lock);
+ rcu_read_unlock();

/*
* We can flush all the TLBs out of the mmu lock without TLB
diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 2926152..33f313b 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -117,4 +117,10 @@ static inline bool permission_fault(struct kvm_mmu *mmu, unsigned pte_access,
}

void kvm_mmu_invalidate_zap_all_pages(struct kvm *kvm);
+
+void kvm_mmu_slot_remove_write_access_lockless(struct kvm *kvm, int slot);
+void
+kvm_mmu_write_protect_pt_masked_lockless(struct kvm *kvm,
+ struct kvm_memory_slot *slot,
+ gfn_t gfn_offset, unsigned long mask);
#endif
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4ac3a27..c6233e1 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3554,8 +3554,7 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log)
dirty_bitmap_buffer = dirty_bitmap + n / sizeof(long);
memset(dirty_bitmap_buffer, 0, n);

- spin_lock(&kvm->mmu_lock);
-
+ rcu_read_lock();
for (i = 0; i < n / sizeof(long); i++) {
unsigned long mask;
gfn_t offset;
@@ -3579,10 +3578,10 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log)
dirty_bitmap_buffer[i] = mask;

offset = i * BITS_PER_LONG;
- kvm_mmu_write_protect_pt_masked(kvm, memslot, offset, mask);
+ kvm_mmu_write_protect_pt_masked_lockless(kvm, memslot,
+ offset, mask);
}
-
- spin_unlock(&kvm->mmu_lock);
+ rcu_read_unlock();

/*
* All the TLBs can be flushed out of mmu lock, see the comments in
@@ -7246,7 +7245,7 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
* See the comments in fast_page_fault().
*/
if ((change != KVM_MR_DELETE) && (mem->flags & KVM_MEM_LOG_DIRTY_PAGES))
- kvm_mmu_slot_remove_write_access(kvm, mem->slot);
+ kvm_mmu_slot_remove_write_access_lockless(kvm, mem->slot);
}

void kvm_arch_flush_shadow_all(struct kvm *kvm)
--
1.8.1.4