[PATCH v2] KVM: x86: enable dirty log gradually in small chunks

Jay Zhou
2020-02-20T04:28:45Z

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@huawei.com>
---
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 | 23 +++++++++++++++++------
arch/x86/kvm/mmu/mmu.c | 8 ++++++--
arch/x86/kvm/vmx/vmx.c | 3 ++-
include/linux/kvm_host.h | 7 ++++++-
tools/testing/selftests/kvm/dirty_log_test.c | 3 ++-
virt/kvm/kvm_main.c | 25 ++++++++++++++++++-------
6 files changed, 51 insertions(+), 18 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 97a72a5..1afd310 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -1267,9 +1267,11 @@ pages in the host.
The flags field supports two flags: KVM_MEM_LOG_DIRTY_PAGES and
KVM_MEM_READONLY. The former can be set to instruct KVM to keep track of
writes to memory within the slot. See KVM_GET_DIRTY_LOG ioctl to know how to
-use it. The latter can be set, if KVM_CAP_READONLY_MEM capability allows it,
-to make a new slot read-only. In this case, writes to this memory will be
-posted to userspace as KVM_EXIT_MMIO exits.
+use it. It will be different if the KVM_DIRTY_LOG_INITIALLY_SET flag of
+KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 is set. For more information, see the
+description of the capability. The latter can be set, if KVM_CAP_READONLY_MEM
+capability allows it, to make a new slot read-only. In this case, writes to
+this memory will be posted to userspace as KVM_EXIT_MMIO exits.

When the KVM_CAP_SYNC_MMU capability is available, changes in the backing of
the memory region are automatically reflected into the guest. For example, an
@@ -5704,10 +5706,19 @@ 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
+Valid flags are::
+
+ #define KVM_DIRTY_LOG_MANUAL_PROTECT (1 << 0)
+ #define KVM_DIRTY_LOG_INITIALLY_SET (1 << 1)
+
+With KVM_DIRTY_LOG_MANUAL_PROTECT 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.
+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 ioctl. However,
+the KVM_DIRTY_LOG_INITIALLY_SET depends on KVM_DIRTY_LOG_MANUAL_PROTECT, 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 +5727,7 @@ 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/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 87e9ba2..f9c120e 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5865,8 +5865,12 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm,
bool flush;

spin_lock(&kvm->mmu_lock);
- flush = slot_handle_all_level(kvm, memslot, slot_rmap_write_protect,
- false);
+ if (kvm->manual_dirty_log_protect & KVM_DIRTY_LOG_INITIALLY_SET)
+ flush = slot_handle_large_level(kvm, memslot,
+ slot_rmap_write_protect, false);
+ else
+ flush = slot_handle_all_level(kvm, memslot,
+ slot_rmap_write_protect, false);
spin_unlock(&kvm->mmu_lock);

/*
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 3be25ec..fcc585a 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_protect & KVM_DIRTY_LOG_INITIALLY_SET))
+ kvm_mmu_slot_leaf_clear_dirty(kvm, slot);
kvm_mmu_slot_largepage_remove_write_access(kvm, slot);
}

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index e89eb67..a555b52 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -39,6 +39,11 @@
#define KVM_MAX_VCPU_ID KVM_MAX_VCPUS
#endif

+#define KVM_DIRTY_LOG_MANUAL_PROTECT (1 << 0)
+#define KVM_DIRTY_LOG_INITIALLY_SET (1 << 1)
+#define KVM_DIRTY_LOG_MANUAL_CAPS (KVM_DIRTY_LOG_MANUAL_PROTECT | \
+ KVM_DIRTY_LOG_INITIALLY_SET)
+
/*
* The bit 16 ~ bit 31 of kvm_memory_region::flags are internally used
* in kvm, other bits are visible for userspace which are defined in
@@ -493,7 +498,7 @@ 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/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
index 5614222..2a493c1 100644
--- a/tools/testing/selftests/kvm/dirty_log_test.c
+++ b/tools/testing/selftests/kvm/dirty_log_test.c
@@ -317,10 +317,11 @@ static void run_test(enum vm_guest_mode mode, unsigned long iterations,
host_bmap_track = bitmap_alloc(host_num_pages);

#ifdef USE_CLEAR_DIRTY_LOG
+ int ret = kvm_check_cap(KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2);
struct kvm_enable_cap cap = {};

cap.cap = KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2;
- cap.args[0] = 1;
+ cap.args[0] = ret;
vm_enable_cap(vm, &cap);
#endif

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 70f03ce..f2631d0 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -862,7 +862,7 @@ static int kvm_vm_release(struct inode *inode, struct file *filp)
* 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 +1094,11 @@ 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_protect & KVM_DIRTY_LOG_INITIALLY_SET)
+ bitmap_set(new.dirty_bitmap, 0, new.npages);
}

slots = kvzalloc(sizeof(struct kvm_memslots), GFP_KERNEL_ACCOUNT);
@@ -1255,7 +1258,7 @@ int kvm_get_dirty_log_protect(struct kvm *kvm,

n = kvm_dirty_bitmap_bytes(memslot);
*flush = false;
- if (kvm->manual_dirty_log_protect) {
+ if (kvm->manual_dirty_log_protect & KVM_DIRTY_LOG_MANUAL_PROTECT) {
/*
* Unlike kvm_get_dirty_log, we always return false in *flush,
* because no flush is needed until KVM_CLEAR_DIRTY_LOG. There
@@ -3310,9 +3313,6 @@ 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 +3320,10 @@ 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;
@@ -3348,7 +3352,14 @@ 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))
+ if (cap->flags ||
+ (cap->args[0] & ~KVM_DIRTY_LOG_MANUAL_CAPS) ||
+ /* The capability of KVM_DIRTY_LOG_INITIALLY_SET depends
+ * on KVM_DIRTY_LOG_MANUAL_PROTECT, it should not be
+ * set individually
+ */
+ ((cap->args[0] & KVM_DIRTY_LOG_MANUAL_CAPS) ==
+ KVM_DIRTY_LOG_INITIALLY_SET))
return -EINVAL;
kvm->manual_dirty_log_protect = cap->args[0];
return 0;
--
1.8.3.1