Subject: Re: [PATCH v2] KVM/arm64: Support enabling dirty log gradually in small chunks

From: Marc Zyngier <[email-masked]>
Date: Thu, 16 Apr 2020 16:08:33 +0100

On Mon, 13 Apr 2020 20:20:23 +0800
Keqian Zhu <[email-masked]> wrote:

> There is already support of enabling dirty log graually in small chunks

gradually

> for x86 in commit 3c9bd4006bfc ("KVM: x86: enable dirty log gradually in
> small chunks"). This adds support for arm64.
> 
> x86 still writes protect all huge pages when DIRTY_LOG_INITIALLY_ALL_SET
> is eanbled. However, for arm64, both huge pages and normal pages can be

enabled

> write protected gradually by userspace.
> 
> Under the Huawei Kunpeng 920 2.6GHz platform, I did some tests on 128G
> Linux VMs with different page size. The memory pressure is 127G in each
> case. The time taken of memory_global_dirty_log_start in QEMU is listed
> below:
> 
> Page Size      Before    After Optimization
>   4K            650ms         1.8ms
>   2M             4ms          1.8ms
>   1G             2ms          1.8ms

These numbers are different from what you have advertised before. What
changed?

> 
> Besides the time reduction, the biggest income is that we will minimize

s/income/improvement/

> the performance side effect (because of dissloving huge pages and marking

dissolving

> memslots dirty) on guest after enabling dirty log.
> 
> Signed-off-by: Keqian Zhu <[email-masked]>
> ---
>  Documentation/virt/kvm/api.rst    |  2 +-
>  arch/arm64/include/asm/kvm_host.h |  3 +++
>  virt/kvm/arm/mmu.c                | 12 ++++++++++--
>  3 files changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index efbbe570aa9b..0017f63fa44f 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -5777,7 +5777,7 @@ will be initialized to 1 when created.  This also improves performance because
>  dirty logging can be enabled gradually in small chunks on the first call
>  to KVM_CLEAR_DIRTY_LOG.  KVM_DIRTY_LOG_INITIALLY_SET depends on
>  KVM_DIRTY_LOG_MANUAL_PROTECT_ENABLE (it is also only available on
> -x86 for now).
> +x86 and arm64 for now).
>  
>  KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 was previously available under the name
>  KVM_CAP_MANUAL_DIRTY_LOG_PROTECT, but the implementation had bugs that make
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 32c8a675e5a4..a723f84fab83 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -46,6 +46,9 @@
>  #define KVM_REQ_RECORD_STEAL	KVM_ARCH_REQ(3)
>  #define KVM_REQ_RELOAD_GICv4	KVM_ARCH_REQ(4)
>  
> +#define KVM_DIRTY_LOG_MANUAL_CAPS   (KVM_DIRTY_LOG_MANUAL_PROTECT_ENABLE | \
> +				     KVM_DIRTY_LOG_INITIALLY_SET)
> +
>  DECLARE_STATIC_KEY_FALSE(userspace_irqchip_in_use);
>  
>  extern unsigned int kvm_sve_max_vl;
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index e3b9ee268823..1077f653a611 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -2265,8 +2265,16 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
>  	 * allocated dirty_bitmap[], dirty pages will be be tracked while the
>  	 * memory slot is write protected.
>  	 */
> -	if (change != KVM_MR_DELETE && mem->flags & KVM_MEM_LOG_DIRTY_PAGES)
> -		kvm_mmu_wp_memory_region(kvm, mem->slot);
> +	if (change != KVM_MR_DELETE && mem->flags & KVM_MEM_LOG_DIRTY_PAGES) {
> +		/*
> +		 * If we're with initial-all-set, we don't need to write
> +		 * protect any pages because they're all reported as dirty.
> +		 * Huge pages and normal pages will be write protect gradually.
> +		 */
> +		if (!kvm_dirty_log_manual_protect_and_init_set(kvm)) {
> +			kvm_mmu_wp_memory_region(kvm, mem->slot);
> +		}
> +	}
>  }
>  
>  int kvm_arch_prepare_memory_region(struct kvm *kvm,

As it is, it is pretty good. The one thing that isn't clear to me is
why we have a difference in behaviour between x86 and arm64. What
prevents x86 from having the same behaviour as arm64?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...

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