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

Peter Xu
2020-02-20T19:17:16Z

On Thu, Feb 20, 2020 at 12:28:28PM +0800, Jay Zhou wrote:
> 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.

Not sure about others, but my own preference is that we keep this part
untouched... The changed document could be even more confusing to me...

>
> 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.

Need s/KVM_DIRTY_LOG_MANUAL_PROTECT/KVM_DIRTY_LOG_MANUAL_PROTECT2/?

>
> 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;

You enabled the initial-all-set but didn't really check it, so it
didn't help much from the testcase pov... I'd suggest you drop this
change, and you can work on top after this patch can be accepted.

(Not to mention the original test actually verified that we don't
break, which seems good..)

> 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)

This change seems irrelevant..

> {
> 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))

Same here.

> goto out_free;
> +
> + if (kvm->manual_dirty_log_protect & KVM_DIRTY_LOG_INITIALLY_SET)

(Maybe time to introduce a helper to shorten this check. :)

> + 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) {

Can also introduce a helper for this too.

Side note: logically this line doesn't need a change, because bit 1
depends on bit 0 so if (kvm->manual_dirty_log_protect) it means bit 0
must be set.

> /*
> * 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))

How about something easier to read? :)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 70f03ce0e5c1..9dfbab2a9929 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3348,7 +3348,10 @@ 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] & ~3))
+ return -EINVAL;
+ /* Allow 00, 01, and 11. */
+ if (cap->args[0] == KVM_DIRTY_LOG_INITIALLY_SET)
return -EINVAL;
kvm->manual_dirty_log_protect = cap->args[0];
return 0;

Otherwise it looks good to me!

Thanks,

--
Peter Xu