RE: [PATCH v3] KVM: x86: enable dirty log gradually in small chunks

["Zhoujian (jay)" <jianjay.zhou@huawei.com>]

> -----Original Message-----
> From: Peter Xu [mailto:peterx@redhat.com]
> Sent: Tuesday, February 25, 2020 1:06 AM
> To: Zhoujian (jay) <jianjay.zhou@huawei.com>
> Cc: kvm@vger.kernel.org; pbonzini@redhat.com;
> sean.j.christopherson@intel.com; wangxin (U) <wangxinxin.wang@huawei.com>;
> Huangweidong (C) <weidong.huang@huawei.com>; Liujinsong (Paul)
> <liu.jinsong@huawei.com>
> Subject: Re: [PATCH v3] KVM: x86: enable dirty log gradually in small chunks
> 
> On Mon, Feb 24, 2020 at 11:25:58AM +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>
> > ---
> > 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)
> 
> I think I mis-read previously on the old version so my comment was misleading.
> If this is the sub-capability within KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2,
> then I don't think we need to have the ending "2" any more.  How about:

It's OK, :-)

> 
>   KVM_MANUAL_PROTECT_ENABLE
>   KVM_MANUAL_PROTECT_INIT_ALL_SET

I think this naming emphasizes more about "manual protect", and the
original naming emphasizes more about "dirty log". The object of manual protect
and initial-all-set is dirty log, so it seem that the original names are a little more
close to the thing we do.

> > +
> > +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;
> > +}
> 
> Nit: this can be put into kvm_host.h as inlined.

I'm afraid not. I tried to do it, but it can't be compiled through. Since this
function is shared between the kvm and kvm_intel(vmx part) module, it should be
exported.

BTW: how about using kvm_dirty_log_manual_protect and_init_set instead of
kvm_manual_dirty_log_init_set (just an idea, your opinions are welcome).

> 
> > +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;
> 
> We probably can only return the new feature bit when with CONFIG_X86?

How about to define different values in different architectures(see
KVM_USER_MEM_SLOTS as an example), like this:

diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
index c3314b2..383a8ae 100644
--- a/arch/arm/include/asm/kvm_host.h
+++ b/arch/arm/include/asm/kvm_host.h
@@ -23,6 +23,10 @@
 #define KVM_HAVE_ONE_REG
 #define KVM_HALT_POLL_NS_DEFAULT 500000

+#define KVM_DIRTY_LOG_MANUAL_PROTECT (1 << 0)
+#define KVM_DIRTY_LOG_INITIALLY_SET 0
+#define KVM_DIRTY_LOG_MANUAL_CAPS KVM_DIRTY_LOG_MANUAL_PROTECT
+
 #define KVM_VCPU_MAX_FEATURES 2

 #include <kvm/arm_vgic.h>
diff --git a/arch/mips/include/asm/kvm_host.h b/arch/mips/include/asm/kvm_host.h
index 41204a4..503ee17 100644
--- a/arch/mips/include/asm/kvm_host.h
+++ b/arch/mips/include/asm/kvm_host.h
@@ -85,6 +85,10 @@

 #define KVM_HALT_POLL_NS_DEFAULT 500000

+#define KVM_DIRTY_LOG_MANUAL_PROTECT (1 << 0)
+#define KVM_DIRTY_LOG_INITIALLY_SET 0
+#define KVM_DIRTY_LOG_MANUAL_CAPS KVM_DIRTY_LOG_MANUAL_PROTECT
+
 #ifdef CONFIG_KVM_MIPS_VZ
 extern unsigned long GUESTID_MASK;
 extern unsigned long GUESTID_FIRST_VERSION;
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 40a0c0f..ac05172 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -49,6 +49,11 @@

 #define KVM_IRQCHIP_NUM_PINS  KVM_IOAPIC_NUM_PINS

+#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)
+
 /* x86-specific vcpu->requests bit members */
 #define KVM_REQ_MIGRATE_TIMER          KVM_ARCH_REQ(0)
 #define KVM_REQ_REPORT_TPR_ACCESS      KVM_ARCH_REQ(1)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index e89eb67..ebd3e55 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -360,6 +360,18 @@ static inline unsigned long *kvm_second_dirty_bitmap(struct kvm_memory_slot *mem
        return memslot->dirty_bitmap + len / sizeof(*memslot->dirty_bitmap);
 }

+#ifndef KVM_DIRTY_LOG_MANUAL_PROTECT
+#define KVM_DIRTY_LOG_MANUAL_PROTECT 0
+#endif
+#ifndef KVM_DIRTY_LOG_INITIALLY_SET
+#define KVM_DIRTY_LOG_INITIALLY_SET 0
+#endif
+#ifndef KVM_DIRTY_LOG_MANUAL_CAPS
+#define KVM_DIRTY_LOG_MANUAL_CAPS 0
+#endif

> 
> > +#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;
> 
> Same here to check against CONFIG_X86?

Same as the above.



Regards,
Jay Zhou