Re: [PATCH v2 08/12] KVM: arm64: Add KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE

[Gavin Shan <gshan@redhat.com>]

Hi Ricardo,

On 2/7/23 3:58 AM, Ricardo Koller wrote:
> Add a capability for userspace to specify the eager split chunk size.
> The chunk size specifies how many pages to break at a time, using a
> single allocation. Bigger the chunk size, more pages need to be
> allocated ahead of time.
> 
> Suggested-by: Oliver Upton <oliver.upton@linux.dev>
> Signed-off-by: Ricardo Koller <ricarkol@google.com>
> ---
>   Documentation/virt/kvm/api.rst    | 26 ++++++++++++++++++++++++++
>   arch/arm64/include/asm/kvm_host.h |  2 ++
>   arch/arm64/kvm/arm.c              | 22 ++++++++++++++++++++++
>   arch/arm64/kvm/mmu.c              |  3 +++
>   include/uapi/linux/kvm.h          |  1 +
>   5 files changed, 54 insertions(+)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 9807b05a1b57..a9332e331cce 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -8284,6 +8284,32 @@ structure.
>   When getting the Modified Change Topology Report value, the attr->addr
>   must point to a byte where the value will be stored or retrieved from.
>   
> +8.40 KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE
> +---------------------------------------
> +
> +:Capability: KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE
> +:Architectures: arm64
> +:Type: vm
> +:Parameters: arg[0] is the new chunk size.
> +:Returns: 0 on success, -EINVAL if any memslot has been created.
> +
> +This capability sets the chunk size used in Eager Page Splitting.
> +
> +Eager Page Splitting improves the performance of dirty-logging (used
> +in live migrations) when guest memory is backed by huge-pages.  This
> +optimization is enabled by default on arm64. It avoids splitting
> +huge-pages (into PAGE_SIZE pages) on fault, by doing it eagerly when
> +enabling dirty logging (with the KVM_MEM_LOG_DIRTY_PAGES flag for a
> +memory region), or when using KVM_CLEAR_DIRTY_LOG.
> +
> +The chunk size specifies how many pages to break at a time, using a
> +single allocation for each chunk. Bigger the chunk size, more pages
> +need to be allocated ahead of time. A good heuristic is to pick the
> +size of the huge-pages as the chunk size.
> +
> +If the chunk size (arg[0]) is zero, then no eager page splitting is
> +performed. The default value PMD size (e.g., 2M when PAGE_SIZE is 4K).
> +
>   9. Known KVM API problems
>   =========================
>   
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 35a159d131b5..a69a815719cf 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -153,6 +153,8 @@ struct kvm_s2_mmu {
>   	/* The last vcpu id that ran on each physical CPU */
>   	int __percpu *last_vcpu_ran;
>   
> +#define KVM_ARM_EAGER_SPLIT_CHUNK_SIZE_DEFAULT	PMD_SIZE
> +
>   	struct kvm_arch *arch;
>   };
>   
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 9c5573bc4614..c80617ced599 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -101,6 +101,22 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>   		r = 0;
>   		set_bit(KVM_ARCH_FLAG_SYSTEM_SUSPEND_ENABLED, &kvm->arch.flags);
>   		break;
> +	case KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE:
> +		mutex_lock(&kvm->lock);
> +		mutex_lock(&kvm->slots_lock);
> +		/*
> +		 * To keep things simple, allow changing the chunk
> +		 * size only if there are no memslots created.
> +		 */
> +		if (!kvm_are_all_memslots_empty(kvm)) {
> +			r = -EINVAL;
> +		} else {
> +			r = 0;
> +			kvm->arch.mmu.split_page_chunk_size = cap->args[0];
> +		}
> +		mutex_unlock(&kvm->slots_lock);
> +		mutex_unlock(&kvm->lock);
> +		break;
>   	default:
>   		r = -EINVAL;
>   		break;
> @@ -298,6 +314,12 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>   	case KVM_CAP_ARM_PTRAUTH_GENERIC:
>   		r = system_has_full_ptr_auth();
>   		break;
> +	case KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE:
> +		if (kvm)
> +			r = kvm->arch.mmu.split_page_chunk_size;
> +		else
> +			r = KVM_ARM_EAGER_SPLIT_CHUNK_SIZE_DEFAULT;
> +		break;
>   	default:
>   		r = 0;
>   	}
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 812633a75e74..e2ada6588017 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -755,6 +755,9 @@ int kvm_init_stage2_mmu(struct kvm *kvm, struct kvm_s2_mmu *mmu, unsigned long t
>   	for_each_possible_cpu(cpu)
>   		*per_cpu_ptr(mmu->last_vcpu_ran, cpu) = -1;
>   
> +	mmu->split_page_cache.gfp_zero = __GFP_ZERO;
> +	mmu->split_page_chunk_size = KVM_ARM_EAGER_SPLIT_CHUNK_SIZE_DEFAULT;
> +
>   	mmu->pgt = pgt;
>   	mmu->pgd_phys = __pa(pgt->pgd);
>   	return 0;
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 55155e262646..02e05f7918e2 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1175,6 +1175,7 @@ struct kvm_ppc_resize_hpt {
>   #define KVM_CAP_DIRTY_LOG_RING_ACQ_REL 223
>   #define KVM_CAP_S390_PROTECTED_ASYNC_DISABLE 224
>   #define KVM_CAP_DIRTY_LOG_RING_WITH_BITMAP 225
> +#define KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE 226
>   
>   #ifdef KVM_CAP_IRQ_ROUTING
>   
> 

mmu->split_page_cache and mmu->split_page_chunk_size are defined in PATCH[09/12].
I think you need move the definitions to PATCH[08/12] instead. Otherwise, git-bisect
is broken.

Thanks,
Gavin