It could take kvm->mmu_lock for an extended period of time when
enabling dirty log for the first time. The main cost is to clear
all the D-bits of last level SPTEs. This situation can benefit from
manual dirty log protect as well, which can reduce the mmu_lock
time taken. The sequence is like this:
1. Initialize all the bits of the dirty bitmap to 1 when enabling
dirty log for the first time
2. Only write protect the huge pages
3. KVM_GET_DIRTY_LOG returns the dirty bitmap info
4. KVM_CLEAR_DIRTY_LOG will clear D-bit for each of the leaf level
SPTEs gradually in small chunks
Under the Intel(R) Xeon(R) Gold 6152 CPU @ 2.10GHz environment,
I did some tests with a 128G windows VM and counted the time taken
of memory_global_dirty_log_start, here is the numbers:
VM Size Before After optimization
128G 460ms 10ms
Signed-off-by: Jay Zhou <jianjay.zhou@xxxxxxxxxx>
---
v3:
* add kvm_manual_dirty_log_init_set helper, add testcase on top and
keep old behavior for KVM_MEM_READONLY [Peter]
* tweak logic at enabling KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 [Sean, Peter]
v2:
* add new bit to KVM_ENABLE_CAP for KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 [Paolo]
* support non-PML path [Peter]
* delete the unnecessary ifdef and make the initialization of bitmap
more clear [Sean]
* document the new bits and tweak the testcase
Documentation/virt/kvm/api.rst | 16 +++++++++++++---
arch/x86/include/asm/kvm_host.h | 3 ++-
arch/x86/kvm/mmu/mmu.c | 7 ++++---
arch/x86/kvm/vmx/vmx.c | 3 ++-
arch/x86/kvm/x86.c | 18 +++++++++++++++---
include/linux/kvm_host.h | 9 ++++++++-
virt/kvm/kvm_main.c | 30 +++++++++++++++++++++++-------
7 files changed, 67 insertions(+), 19 deletions(-)
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 97a72a5..807fcd7 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -5704,10 +5704,20 @@ and injected exceptions.
:Architectures: x86, arm, arm64, mips
:Parameters: args[0] whether feature should be enabled or not
-With this capability enabled, KVM_GET_DIRTY_LOG will not automatically
-clear and write-protect all pages that are returned as dirty.
+Valid flags are::
+
+ #define KVM_DIRTY_LOG_MANUAL_PROTECT2 (1 << 0)
+ #define KVM_DIRTY_LOG_INITIALLY_SET (1 << 1)
+
+With KVM_DIRTY_LOG_MANUAL_PROTECT2 set, KVM_GET_DIRTY_LOG will not
+automatically clear and write-protect all pages that are returned as dirty.
Rather, userspace will have to do this operation separately using
KVM_CLEAR_DIRTY_LOG.
+With KVM_DIRTY_LOG_INITIALLY_SET set, all the bits of the dirty bitmap
+will be initialized to 1 when created, dirty logging will be enabled
+gradually in small chunks using KVM_CLEAR_DIRTY_LOG. However, the
+KVM_DIRTY_LOG_INITIALLY_SET depends on KVM_DIRTY_LOG_MANUAL_PROTECT2,
+it can not be set individually and supports x86 only for now.
At the cost of a slightly more complicated operation, this provides better
scalability and responsiveness for two reasons. First,
@@ -5716,7 +5726,7 @@ than requiring to sync a full memslot; this ensures that KVM does not
take spinlocks for an extended period of time. Second, in some cases a
large amount of time can pass between a call to KVM_GET_DIRTY_LOG and
userspace actually using the data in the page. Pages can be modified
-during this time, which is inefficint for both the guest and userspace:
+during this time, which is inefficient for both the guest and userspace:
the guest will incur a higher penalty due to write protection faults,
while userspace can see false reports of dirty pages. Manual reprotection
helps reducing this time, improving guest performance and reducing the
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 40a0c0f..a90630c 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1312,7 +1312,8 @@ void kvm_mmu_set_mask_ptes(u64 user_mask, u64 accessed_mask,
void kvm_mmu_reset_context(struct kvm_vcpu *vcpu);
void kvm_mmu_slot_remove_write_access(struct kvm *kvm,
- struct kvm_memory_slot *memslot);
+ struct kvm_memory_slot *memslot,
+ int start_level);
void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
const struct kvm_memory_slot *memslot);
void kvm_mmu_slot_leaf_clear_dirty(struct kvm *kvm,
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 87e9ba2..a4e70eb 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5860,13 +5860,14 @@ static bool slot_rmap_write_protect(struct kvm *kvm,
}
void kvm_mmu_slot_remove_write_access(struct kvm *kvm,
- struct kvm_memory_slot *memslot)
+ struct kvm_memory_slot *memslot,
+ int start_level)
{
bool flush;
spin_lock(&kvm->mmu_lock);
- flush = slot_handle_all_level(kvm, memslot, slot_rmap_write_protect,
- false);
+ flush = slot_handle_level(kvm, memslot, slot_rmap_write_protect,
+ start_level, PT_MAX_HUGEPAGE_LEVEL, false);
spin_unlock(&kvm->mmu_lock);
/*
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 3be25ec..0deb8c3 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7201,7 +7201,8 @@ static void vmx_sched_in(struct kvm_vcpu *vcpu, int cpu)
static void vmx_slot_enable_log_dirty(struct kvm *kvm,
struct kvm_memory_slot *slot)
{
- kvm_mmu_slot_leaf_clear_dirty(kvm, slot);
+ if (!kvm_manual_dirty_log_init_set(kvm))
+ kvm_mmu_slot_leaf_clear_dirty(kvm, slot);
kvm_mmu_slot_largepage_remove_write_access(kvm, slot);
}
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fb5d64e..f816940 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9956,7 +9956,7 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
{
/* Still write protect RO slot */
if (new->flags & KVM_MEM_READONLY) {
- kvm_mmu_slot_remove_write_access(kvm, new);
+ kvm_mmu_slot_remove_write_access(kvm, new, PT_PAGE_TABLE_LEVEL);
return;
}
@@ -9993,8 +9993,20 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
if (new->flags & KVM_MEM_LOG_DIRTY_PAGES) {
if (kvm_x86_ops->slot_enable_log_dirty)
kvm_x86_ops->slot_enable_log_dirty(kvm, new);
- else
- kvm_mmu_slot_remove_write_access(kvm, new);
+ else {
+ int level = kvm_manual_dirty_log_init_set(kvm) ?
+ PT_DIRECTORY_LEVEL : PT_PAGE_TABLE_LEVEL;
+
+ /*
+ * If we're with initial-all-set, we don't need
+ * to write protect any small page because
+ * they're reported as dirty already. However
+ * we still need to write-protect huge pages
+ * so that the page split can happen lazily on
+ * the first write to the huge page.
+ */
+ kvm_mmu_slot_remove_write_access(kvm, new, level);
+ }
} else {
if (kvm_x86_ops->slot_disable_log_dirty)
kvm_x86_ops->slot_disable_log_dirty(kvm, new);
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index e89eb67..80ada94 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -360,6 +360,13 @@ static inline unsigned long *kvm_second_dirty_bitmap(struct kvm_memory_slot *mem
return memslot->dirty_bitmap + len / sizeof(*memslot->dirty_bitmap);
}
+#define KVM_DIRTY_LOG_MANUAL_PROTECT2 (1 << 0)
+#define KVM_DIRTY_LOG_INITIALLY_SET (1 << 1)
+#define KVM_DIRTY_LOG_MANUAL_CAPS (KVM_DIRTY_LOG_MANUAL_PROTECT2 | \
+ KVM_DIRTY_LOG_INITIALLY_SET)
+
+bool kvm_manual_dirty_log_init_set(struct kvm *kvm);
+
struct kvm_s390_adapter_int {
u64 ind_addr;
u64 summary_addr;
@@ -493,7 +500,7 @@ struct kvm {
#endif
long tlbs_dirty;
struct list_head devices;
- bool manual_dirty_log_protect;
+ u64 manual_dirty_log_protect;
struct dentry *debugfs_dentry;
struct kvm_stat_data **debugfs_stat_data;
struct srcu_struct srcu;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 70f03ce..0ffb804 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -858,11 +858,17 @@ static int kvm_vm_release(struct inode *inode, struct file *filp)
return 0;
}
+bool kvm_manual_dirty_log_init_set(struct kvm *kvm)
+{
+ return kvm->manual_dirty_log_protect & KVM_DIRTY_LOG_INITIALLY_SET;
+}
+EXPORT_SYMBOL_GPL(kvm_manual_dirty_log_init_set);
+
/*
* Allocation size is twice as large as the actual dirty bitmap size.
* See x86's kvm_vm_ioctl_get_dirty_log() why this is needed.
*/
-static int kvm_create_dirty_bitmap(struct kvm_memory_slot *memslot)
+static int kvm_alloc_dirty_bitmap(struct kvm_memory_slot *memslot)
{
unsigned long dirty_bytes = 2 * kvm_dirty_bitmap_bytes(memslot);
@@ -1094,8 +1100,11 @@ int __kvm_set_memory_region(struct kvm *kvm,
/* Allocate page dirty bitmap if needed */
if ((new.flags & KVM_MEM_LOG_DIRTY_PAGES) && !new.dirty_bitmap) {
- if (kvm_create_dirty_bitmap(&new) < 0)
+ if (kvm_alloc_dirty_bitmap(&new))
goto out_free;
+
+ if (kvm_manual_dirty_log_init_set(kvm))
+ bitmap_set(new.dirty_bitmap, 0, new.npages);
}
slots = kvzalloc(sizeof(struct kvm_memslots), GFP_KERNEL_ACCOUNT);
@@ -3310,9 +3319,6 @@ static long kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
case KVM_CAP_IOEVENTFD_ANY_LENGTH:
case KVM_CAP_CHECK_EXTENSION_VM:
case KVM_CAP_ENABLE_CAP_VM:
-#ifdef CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT
- case KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2:
-#endif
return 1;
#ifdef CONFIG_KVM_MMIO
case KVM_CAP_COALESCED_MMIO:
@@ -3320,6 +3326,10 @@ static long kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
case KVM_CAP_COALESCED_PIO:
return 1;
#endif
+#ifdef CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT
+ case KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2:
+ return KVM_DIRTY_LOG_MANUAL_CAPS;
+#endif
#ifdef CONFIG_HAVE_KVM_IRQ_ROUTING
case KVM_CAP_IRQ_ROUTING:
return KVM_MAX_IRQ_ROUTES;
@@ -3347,11 +3357,17 @@ static int kvm_vm_ioctl_enable_cap_generic(struct kvm *kvm,
{
switch (cap->cap) {
#ifdef CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT
- case KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2:
- if (cap->flags || (cap->args[0] & ~1))
+ case KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2: {
+ u64 allowed_options = KVM_DIRTY_LOG_MANUAL_PROTECT2;
+
+ if (cap->args[0] & KVM_DIRTY_LOG_MANUAL_PROTECT2)
+ allowed_options |= KVM_DIRTY_LOG_INITIALLY_SET;
+
+ if (cap->flags || (cap->args[0] & ~allowed_options))
return -EINVAL;
kvm->manual_dirty_log_protect = cap->args[0];
return 0;
+ }
#endif
default:
return kvm_vm_ioctl_enable_cap(kvm, cap);
--
1.8.3.1
