[PATCH 0/7] KVM: MMU: fast write protect

From: guangrong . xiao
Date: Wed May 03 2017 - 06:53:15 EST

From: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxx>

The original idea of this patchset is from Avi who raised it in
the mailing list during my vMMU development some years ago

This patchset introduces a extremely fast way to write protect
all the guest memory. Comparing with the ordinary algorithm which
write protects last level sptes based on the rmap one by one,
it just simply updates the generation number to ask all vCPUs
to reload its root page table, particularly, it can be done out
of mmu-lock, so that it does not hurt vMMU's parallel. It is
the O(1) algorithm which does not depends on the capacity of
guest's memory and the number of guest's vCPUs

When write protect for all guest memory is required, we update
the global generation number and ask vCPUs to reload its root
page table by calling kvm_reload_remote_mmus(), the global number
is protected by slots_lock

During reloading its root page table, the vCPU checks root page
table's generation number with current global number, if it is not
matched, it makes all the entries in the shadow page readonly and
directly go to VM. So the read access is still going on smoothly
without KVM's involvement and write access triggers page fault

If the page fault is triggered by write operation, KVM moves the
write protection from the upper level to the lower level page - by
making all the entries in the lower page readonly first then make
the upper level writable, this operation is repeated until we meet
the last spte

In order to speed up the process of making all entries readonly, we
introduce possible_writable_spte_bitmap which indicates the writable
sptes and possiable_writable_sptes which is a counter indicating the
number of writable sptes in the shadow page, they work very efficiently
as usually only one entry in PML4 ( < 512 G)ïfew entries in PDPT (one
entry indicates 1G memory), PDEs and PTEs need to be write protected for
the worst case. Note, the number of page fault and TLB flush are the same
as the ordinary algorithm

Performance Data
Case 1) For a VM which has 3G memory and 12 vCPUs, we noticed that:
a: the time required for dirty log (ns)
before after
64289121 137654 +46603%

b: the performance of memory write after dirty log, i.e, the dirty
log path is not parallel with page fault, the time required to
write all 3G memory for all vCPUs in the VM (ns):
before after
281735017 291150923 -3%
We think the impact, 3%, is acceptable, particularly, mmu-lock
contention is not take into account in this case

Case 2) For a VM which has 30G memory and 8 vCPUs, we do the live
migration, at the some time, a test case which greedily and repeatedly
writes 3000M memory in the VM.

2.1) for the new booted VM, i.e, page fault is required to map guest
memory in, we noticed that:
a: the dirty page rate (pages):
before after
333092 497266 +49%
that means, the performance for the being migrated VM is hugely
improved as the contention on mmu-lock is reduced

b: the time to complete live migration (ms):
before after
12532 18467 -47%
not surprise, the time required to complete live migration is
increased as the VM is able to generate more dirty pages

2.2) pre-write the VM first, then run the test case and do live
migration, i.e, no much page faults are needed to map guest
memory in, we noticed that:
a: the dirty page rate (pages):
before after
447435 449284 +0%

b: time time to complete live migration (ms)
before after
31068 28310 +10%
under this case, we also noticed that the time of dirty log for
the first time, before the patchset is 156 ms, after that, only
6 ms is needed

The patch applied to QEMU
The draft patch is attached to enable this functionality in QEMU:

diff --git a/kvm-all.c b/kvm-all.c
index 90b8573..9ebe1ac 100644
--- a/kvm-all.c
+++ b/kvm-all.c
@@ -122,6 +122,7 @@ bool kvm_direct_msi_allowed;
bool kvm_ioeventfd_any_length_allowed;
bool kvm_msi_use_devid;
static bool kvm_immediate_exit;
+static bool kvm_write_protect_all;

static const KVMCapabilityInfo kvm_required_capabilites[] = {
@@ -440,6 +441,26 @@ static int kvm_get_dirty_pages_log_range(MemoryRegionSection *section,

#define ALIGN(x, y) (((x)+(y)-1) & ~((y)-1))

+static bool kvm_write_protect_all_is_supported(KVMState *s)
+ return kvm_check_extension(s, KVM_CAP_X86_WRITE_PROTECT_ALL_MEM) &&
+ kvm_check_extension(s, KVM_CAP_X86_DIRTY_LOG_WITHOUT_WRITE_PROTECT);
+static void kvm_write_protect_all_mem(bool write)
+ int ret;
+ if (!kvm_write_protect_all)
+ return;
+ ret = kvm_vm_ioctl(kvm_state, KVM_WRITE_PROTECT_ALL_MEM, !!write);
+ if (ret < 0) {
+ printf("ioctl failed %d\n", errno);
+ abort();
+ }
* kvm_physical_sync_dirty_bitmap - Grab dirty bitmap from kernel space
* This function updates qemu's dirty bitmap using
@@ -490,6 +511,7 @@ static int kvm_physical_sync_dirty_bitmap(KVMMemoryListener *kml,
memset(d.dirty_bitmap, 0, allocated_size);

d.slot = mem->slot | (kml->as_id << 16);
+ d.flags = kvm_write_protect_all ? KVM_DIRTY_LOG_WITHOUT_WRITE_PROTECT : 0;
if (kvm_vm_ioctl(s, KVM_GET_DIRTY_LOG, &d) == -1) {
DPRINTF("ioctl failed %d\n", errno);
ret = -1;
@@ -1622,6 +1644,9 @@ static int kvm_init(MachineState *ms)

kvm_immediate_exit = kvm_check_extension(s, KVM_CAP_IMMEDIATE_EXIT);
+ kvm_write_protect_all = kvm_write_protect_all_is_supported(s);
+ printf("Write protect all is %s.\n", kvm_write_protect_all ? "supported" : "unsupported");
+ memory_register_write_protect_all(kvm_write_protect_all_mem);
s->nr_slots = kvm_check_extension(s, KVM_CAP_NR_MEMSLOTS);

/* If unspecified, use the default value */
diff --git a/linux-headers/linux/kvm.h b/linux-headers/linux/kvm.h
index 4e082a8..7c056ef 100644
--- a/linux-headers/linux/kvm.h
+++ b/linux-headers/linux/kvm.h
@@ -443,9 +443,12 @@ struct kvm_interrupt {

struct kvm_dirty_log {
__u32 slot;
- __u32 padding1;
+ __u32 flags;
union {
void *dirty_bitmap; /* one bit per page */
__u64 padding2;
@@ -884,6 +887,9 @@ struct kvm_ppc_resize_hpt {
#define KVM_CAP_PPC_MMU_HASH_V3 135


struct kvm_irq_routing_irqchip {
@@ -1126,6 +1132,7 @@ enum kvm_device_type {
struct kvm_userspace_memory_region)
#define KVM_SET_TSS_ADDR _IO(KVMIO, 0x47)
#define KVM_SET_IDENTITY_MAP_ADDR _IOW(KVMIO, 0x48, __u64)

/* enable ucontrol for s390 */
struct kvm_s390_ucas_mapping {
diff --git a/memory.c b/memory.c
index 4c95aaf..b836675 100644
--- a/memory.c
+++ b/memory.c
@@ -809,6 +809,13 @@ static void address_space_update_ioeventfds(AddressSpace *as)

+static write_protect_all_fn write_func;
+void memory_register_write_protect_all(write_protect_all_fn func)
+ printf("Write function is being registering...\n");
+ write_func = func;
static void address_space_update_topology_pass(AddressSpace *as,
const FlatView *old_view,
const FlatView *new_view,
@@ -859,6 +866,8 @@ static void address_space_update_topology_pass(AddressSpace *as,
MEMORY_LISTENER_UPDATE_REGION(frnew, as, Reverse, log_stop,
+ if (write_func)
+ write_func(false);

@@ -2267,6 +2276,9 @@ void memory_global_dirty_log_sync(void)
+ if (write_func)
+ write_func(true);

Xiao Guangrong (7):
KVM: MMU: correct the behavior of mmu_spte_update_no_track
KVM: MMU: introduce possible_writable_spte_bitmap
KVM: MMU: introduce kvm_mmu_write_protect_all_pages
KVM: MMU: allow dirty log without write protect
KVM: MMU: clarify fast_pf_fix_direct_spte
KVM: MMU: stop using mmu_spte_get_lockless under mmu-lock

arch/x86/include/asm/kvm_host.h | 25 +++-
arch/x86/kvm/mmu.c | 267 ++++++++++++++++++++++++++++++++++++++--
arch/x86/kvm/mmu.h | 1 +
arch/x86/kvm/paging_tmpl.h | 13 +-
arch/x86/kvm/x86.c | 7 ++
include/uapi/linux/kvm.h | 8 +-
virt/kvm/kvm_main.c | 15 ++-
7 files changed, 317 insertions(+), 19 deletions(-)