On Mon, Apr 17, 2023 at 03:04:47PM +0800, Gavin Shan wrote:
> On 4/9/23 2:29 PM, 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 | 28 ++++++++++++++++++++++++++
> > arch/arm64/include/asm/kvm_host.h | 15 ++++++++++++++
> > arch/arm64/include/asm/kvm_pgtable.h | 18 +++++++++++++++++
> > arch/arm64/kvm/arm.c | 30 ++++++++++++++++++++++++++++
> > arch/arm64/kvm/mmu.c | 3 +++
> > include/uapi/linux/kvm.h | 2 ++
> > 6 files changed, 96 insertions(+)
> >
>
> With the following comments addressed:
>
> Reviewed-by: Gavin Shan <gshan@xxxxxxxxxx>
>
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index 62de0768d6aa5..f8faa80d87057 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -8380,6 +8380,34 @@ 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 split chunk size.
> > +:Returns: 0 on success, -EINVAL if any memslot was already created.
> ^^^^^^^^^^^^^^^^^^^
>
> Maybe s/was already created/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. 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.
> > +
> > +The chunk size needs to be a valid block size. The list of acceptable
> > +block sizes is exposed in KVM_CAP_ARM_SUPPORTED_BLOCK_SIZES as a 64bit
> > +bitmap (each bit describing a block size). Setting
> > +KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE to 0 disables Eager Page Splitting;
> > +this is the default value.
> > +
>
> s/a 64bit bitmap/a 64-bit bitmap
>
> For the last sentence, maybe:
>
> The default value is 0, to disable the eager page splitting.
Fixed, much better.
>
> > 9. Known KVM API problems
> > =========================
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index a1892a8f60323..b87da1ebc3454 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -158,6 +158,21 @@ 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 0
> > + /*
> > + * 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
> > + * influences both the capacity of the split page cache, and
> > + * how often KVM reschedules. Be wary of raising CHUNK_SIZE
> > + * too high.
> > + *
> > + * 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/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> > index 32e5d42bf020f..889bd7afeb355 100644
> > --- a/arch/arm64/include/asm/kvm_pgtable.h
> > +++ b/arch/arm64/include/asm/kvm_pgtable.h
> > @@ -92,6 +92,24 @@ static inline bool kvm_level_supports_block_mapping(u32 level)
> > return level >= KVM_PGTABLE_MIN_BLOCK_LEVEL;
> > }
> > +static inline u64 kvm_supported_block_sizes(void)
> > +{
> > + u32 level = KVM_PGTABLE_MIN_BLOCK_LEVEL;
> > + u64 res = 0;
> > +
> > + for (; level < KVM_PGTABLE_MAX_LEVELS; level++)
> > + res |= BIT(kvm_granule_shift(level));
> > +
> > + return res;
> > +}
> > +
>
> maybe s/@res/@r
changed
>
> > +static inline bool kvm_is_block_size_supported(u64 size)
> > +{
> > + bool is_power_of_two = !((size) & ((size)-1));
> > +
> > + return is_power_of_two && (size & kvm_supported_block_sizes());
> > +}
> > +
>
> IS_ALIGNED() maybe used here.
I've been trying to reuse some bitmap related function in the kernel,
like IS_ALIGNED(), but can't find anything. Or at least it doesn't occur
to me how.
kvm_is_block_size_supported() returns true if @size matches only one of
the bits set in kvm_supported_block_sizes(). For example, given these
supported sizes: 10000100001000.
kvm_is_block_size_supported(100000000) => true
kvm_is_block_size_supported(1100) => false
>
> > /**
> > * struct kvm_pgtable_mm_ops - Memory management callbacks.
> > * @zalloc_page: Allocate a single zeroed memory page.
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 3bd732eaf0872..34fd3c59a9b82 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -67,6 +67,7 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> > struct kvm_enable_cap *cap)
> > {
> > int r;
> > + u64 new_cap;
> > if (cap->flags)
> > return -EINVAL;
> > @@ -91,6 +92,26 @@ 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:
> > + new_cap = cap->args[0];
> > +
> > + mutex_lock(&kvm->lock);
> > + mutex_lock(&kvm->slots_lock);
> > + /*
> > + * To keep things simple, allow changing the chunk
> > + * size only if there are no memslots already created.
> > + */
>
> /*
> * To keep things simple, allow changing the chunk size
> * only when no memory slots have been created.
> */
>
> > + if (!kvm_are_all_memslots_empty(kvm)) {
> > + r = -EINVAL;
> > + } else if (new_cap && !kvm_is_block_size_supported(new_cap)) {
> > + r = -EINVAL;
> > + } else {
> > + r = 0;
> > + kvm->arch.mmu.split_page_chunk_size = new_cap;
> > + }
> > + mutex_unlock(&kvm->slots_lock);
> > + mutex_unlock(&kvm->lock);
> > + break;
> > default:
> > r = -EINVAL;
> > break;
> > @@ -288,6 +309,15 @@ 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;
> > + case KVM_CAP_ARM_SUPPORTED_BLOCK_SIZES:
> > + r = kvm_supported_block_sizes();
> > + break;
>
> kvm_supported_block_sizes() returns u64, but @r is 32-bits in width. It may be
> worthy to make the return value from kvm_supported_block_sizes() as u32.
>
> > default:
> > r = 0;
> > }
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index a2800e5c42712..898985b09321a 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;
>
> It may be worthy to have comments like below:
>
> /* The eager page splitting is disabled by default */
> > + 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 d77aef872a0a0..f18b48fcd25ba 100644
> > --- a/include/uapi/linux/kvm.h
> > +++ b/include/uapi/linux/kvm.h
> > @@ -1184,6 +1184,8 @@ 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
> > +#define KVM_CAP_ARM_SUPPORTED_BLOCK_SIZES 228
> > #ifdef KVM_CAP_IRQ_ROUTING
> >
>
> Thanks,
> Gavin
>
