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

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Tue, Feb 18, 2020 at 07:00:13PM +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. Set all the bits of the first 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>
> ---
>  arch/x86/kvm/vmx/vmx.c   |  5 +++++
>  include/linux/kvm_host.h |  5 +++++
>  virt/kvm/kvm_main.c      | 10 ++++++++--
>  3 files changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 3be25ec..a8d64f6 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7201,7 +7201,12 @@ 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)
>  {
> +#if CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT
> +	if (!kvm->manual_dirty_log_protect)
> +		kvm_mmu_slot_leaf_clear_dirty(kvm, slot);
> +#else
>  	kvm_mmu_slot_leaf_clear_dirty(kvm, slot);
> +#endif

The ifdef is unnecessary, this is in VMX (x86) code, i.e.
CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT is guaranteed to be defined.

>  	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..fd149b0 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -360,6 +360,11 @@ static inline unsigned long *kvm_second_dirty_bitmap(struct kvm_memory_slot *mem
>  	return memslot->dirty_bitmap + len / sizeof(*memslot->dirty_bitmap);
>  }
>  
> +static inline void kvm_set_first_dirty_bitmap(struct kvm_memory_slot *memslot)
> +{
> +	bitmap_set(memslot->dirty_bitmap, 0, memslot->npages);
> +}

I'd prefer this be open coded with a comment, e.g. "first" is misleading
because it's really "initial dirty bitmap for this memslot after enabling
dirty logging".

> +
>  struct kvm_s390_adapter_int {
>  	u64 ind_addr;
>  	u64 summary_addr;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 70f03ce..08565ed 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -862,7 +862,8 @@ 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_create_dirty_bitmap(struct kvm *kvm,
> +				struct kvm_memory_slot *memslot)
>  {
>  	unsigned long dirty_bytes = 2 * kvm_dirty_bitmap_bytes(memslot);
>  
> @@ -870,6 +871,11 @@ static int kvm_create_dirty_bitmap(struct kvm_memory_slot *memslot)
>  	if (!memslot->dirty_bitmap)
>  		return -ENOMEM;
>  
> +#if CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT

The ifdef is unnecessary, manual_dirty_log_protect always exists and is
guaranteed to be false if CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT=n.  This
isn't exactly a hot path so saving the uop isn't worth the #ifdef.

> +	if (kvm->manual_dirty_log_protect)
> +		kvm_set_first_dirty_bitmap(memslot);
> +#endif
> +
>  	return 0;
>  }
>  
> @@ -1094,7 +1100,7 @@ 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_create_dirty_bitmap(kvm, &new) < 0)

Rather than pass @kvm, what about doing bitmap_set() in __kvm_set_memory_region()
and s/kvm_create_dirty_bitmap/kvm_alloc_dirty_bitmap to make it clear that
the helper is only responsible for allocation?  And opportunistically drop
the superfluous "< 0", e.g.

	if ((new.flags & KVM_MEM_LOG_DIRTY_PAGES) && !new.dirty_bitmap) {
		if (kvm_alloc_dirty_bitmap(&new))
			goto out_free;

		/*
		 * WORDS!
		 */		
		if (kvm->manual_dirty_log_protect)
			bitmap_set(memslot->dirty_bitmap, 0, memslot->npages);
	}

>  			goto out_free;
>  	}
>  
> -- 
> 1.8.3.1
> 
>