On Sun, Mar 12, 2023 at 11:56:27AM +0000, Marc Zyngier wrote:
> On Tue, 07 Mar 2023 03:45:51 +0000,
> Ricardo Koller <ricarkol@xxxxxxxxxx> 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.
>
> split chunk size?
>
> > +:Returns: 0 on success, -EINVAL if any memslot has been created.
>
> nit: if any memslot was *already* 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.
>
> Why enabled by default? It means that systems that do not want to pay
> the extra memory for this have to do an explicit call to disable it.
> I'd rather see this as a buy-in option.
>
Will disable by default.
> > 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.
>
> How about making this a requirement rather than a heuristic?
Sounds good. Planning to return EINVAL for anything that's not a
supported block size.
> You could
> also tell userspace what are the block sizes that are acceptable (1G,
> 512M, 2M, 64K...) by exposing a 64bit bitmap (each bit describing a
> block size).
Good idea, I'm thinking of using a new KVM_CAP_ARM_SUPPORTED_BLOCK_SIZES
capability to return this bitmap.
>
> > +
> > +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).
>
> I really dislike exposing the notion of PMD to userspace. Not only
> this is a concept that is mostly foreign to the arm64 architecture,
> this isn't a userspace concept at all. Another reason to talk about
> block sizes (but I really want this to default to 0 and keep the
> current behaviour as the default).
>
> > +
> > 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
> > + * 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.
>
> This is a 4kB-ness. Nothing "usual" about it (and my 16kB hosts
> definitely object!).
>
> > + *
> > + * Protected by kvm->slots_lock.
> > + */
> > + struct kvm_mmu_memory_cache split_page_cache;
>
> If this is living in kvm_s2_mmu, and that this is a proper memcache,
> why does patch #4 have this horrible 'struct stage2_split_data' that
> uses a 'void *' and carries a kvm_s2_mmu pointer?
>
> Surely, passing the memcache around should be enough (and container_of
> is your forever friend).
>
This simplified things quite a bit.
> > + 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);
> > + 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
> >
>
> Thanks,
>
> M.
>
> --
> Without deviation from the norm, progress is not possible.
Thanks,
Ricardo
