Hi Ricardo,
On Tue, Mar 07, 2023 at 03:45:51AM +0000, 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@xxxxxxxxx>
> Signed-off-by: Ricardo Koller <ricarkol@xxxxxxxxxx>
> ---
> Documentation/virt/kvm/api.rst | 26 ++++++++++++++++++++++++++
> arch/arm64/include/asm/kvm_host.h | 19 +++++++++++++++++++
> arch/arm64/kvm/arm.c | 22 ++++++++++++++++++++++
> arch/arm64/kvm/mmu.c | 3 +++
> include/uapi/linux/kvm.h | 1 +
> 5 files changed, 71 insertions(+)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 62de0768d6aa..872dae7cfbe0 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -8380,6 +8380,32 @@ 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 a1892a8f6032..b7755d0cbd4d 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -158,6 +158,25 @@ 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
> + /*
> + * Memory cache used to split
> + * KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE worth of huge pages. It
> + * is used to allocate stage2 page tables while splitting huge
> + * pages. Note that the choice of EAGER_PAGE_SPLIT_CHUNK_SIZE
Nit: s/EAGER_PAGE_SPLIT_CHUNK_SIZE/EAGER_SPLIT_CHUNK_SIZE/ ?
(or 'split_page_chunk_size' to make the comment consistent
with the field name?)
> + * influences both the capacity of the split page cache, and
> + * how often KVM reschedules. Be wary of raising CHUNK_SIZE
> + * too high.
> + *
> + * A good heuristic to pick CHUNK_SIZE is that it should be
> + * the size of the huge-pages backing guest memory. If not
> + * known, the PMD size (usually 2M) is a good guess.
> + *
> + * Protected by kvm->slots_lock.
> + */
> + struct kvm_mmu_memory_cache split_page_cache;
> + uint64_t split_page_chunk_size;
> +
> struct kvm_arch *arch;
> };
>
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 3bd732eaf087..3468fee223ae 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -91,6 +91,22 @@ 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);
Do we need to hold kvm->lock here ?
Thanks,
Reiji
> + 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;
> @@ -288,6 +304,12 @@ 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 a2800e5c4271..898985b09321 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -756,6 +756,9 @@ 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 d77aef872a0a..af43acdc7901 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1184,6 +1184,7 @@ struct kvm_ppc_resize_hpt {
> #define KVM_CAP_S390_PROTECTED_ASYNC_DISABLE 224
> #define KVM_CAP_DIRTY_LOG_RING_WITH_BITMAP 225
> #define KVM_CAP_PMU_EVENT_MASKED_EVENTS 226
> +#define KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE 227
>
> #ifdef KVM_CAP_IRQ_ROUTING
>
> --
> 2.40.0.rc0.216.gc4246ad0f0-goog
>
