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

[zhukeqian]

Hi Paolo,

On 2020/4/16 23:55, Paolo Bonzini wrote:
> On 16/04/20 17:09, Marc Zyngier wrote:
>> On Wed, 15 Apr 2020 18:13:56 +0200
>> Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>>
>>> On 13/04/20 14:20, Keqian Zhu wrote:
>>>> There is already support of enabling dirty log graually in small chunks
>>>> 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
>>>> 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
>>>>
>>>> Besides the time reduction, the biggest income is that we will minimize
>>>> the performance side effect (because of dissloving huge pages and marking
>>>> memslots dirty) on guest after enabling dirty log.
>>>>
>>>> Signed-off-by: Keqian Zhu <zhukeqian1@xxxxxxxxxx>
>>>> ---
>>>>  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,
>>>>   
>>>
>>> Marc, what is the status of this patch?
>>
>> I just had a look at it. Is there any urgency for merging it?
> 
> No, I thought I was still replying to the v1.
Sorry that patch v1 is dropped. Because I realized that stage2 page tables
will be unmapped during VM reboot, or they are not established soon after
migration, so stage2 page tables can not be used to decide whether a page
is needed to migrate.

Thanks,
Keqian

> 
> Paolo
> 
> 
> .
>