Hi David,
On Mon, May 16, 2022 at 11:21:38PM +0000, David Matlack wrote:
> Add support for Eager Page Splitting pages that are mapped by nested
> MMUs. Walk through the rmap first splitting all 1GiB pages to 2MiB
> pages, and then splitting all 2MiB pages to 4KiB pages.
>
> Note, Eager Page Splitting is limited to nested MMUs as a policy rather
> than due to any technical reason (the sp->role.guest_mode check could
> just be deleted and Eager Page Splitting would work correctly for all
> shadow MMU pages). There is really no reason to support Eager Page
> Splitting for tdp_mmu=N, since such support will eventually be phased
> out, and there is no current use case supporting Eager Page Splitting on
> hosts where TDP is either disabled or unavailable in hardware.
> Furthermore, future improvements to nested MMU scalability may diverge
> the code from the legacy shadow paging implementation. These
> improvements will be simpler to make if Eager Page Splitting does not
> have to worry about legacy shadow paging.
>
> Splitting huge pages mapped by nested MMUs requires dealing with some
> extra complexity beyond that of the TDP MMU:
>
> (1) The shadow MMU has a limit on the number of shadow pages that are
> allowed to be allocated. So, as a policy, Eager Page Splitting
> refuses to split if there are KVM_MIN_FREE_MMU_PAGES or fewer
> pages available.
>
> (2) Splitting a huge page may end up re-using an existing lower level
> shadow page tables. This is unlike the TDP MMU which always allocates
> new shadow page tables when splitting.
>
> (3) When installing the lower level SPTEs, they must be added to the
> rmap which may require allocating additional pte_list_desc structs.
>
> Case (2) is especially interesting since it may require a TLB flush,
> unlike the TDP MMU which can fully split huge pages without any TLB
> flushes. Specifically, an existing lower level page table may point to
> even lower level page tables that are not fully populated, effectively
> unmapping a portion of the huge page, which requires a flush.
>
> This commit performs such flushes after dropping the huge page and
> before installing the lower level page table. This TLB flush could
> instead be delayed until the MMU lock is about to be dropped, which
> would batch flushes for multiple splits. However these flushes should
> be rare in practice (a huge page must be aliased in multiple SPTEs and
> have been split for NX Huge Pages in only some of them). Flushing
> immediately is simpler to plumb and also reduces the chances of tripping
> over a CPU bug (e.g. see iTLB multihit).
>
> Suggested-by: Peter Feiner <pfeiner@xxxxxxxxxx>
> [ This commit is based off of the original implementation of Eager Page
> Splitting from Peter in Google's kernel from 2016. ]
> Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx>
> ---
> .../admin-guide/kernel-parameters.txt | 3 +-
> arch/x86/include/asm/kvm_host.h | 24 ++
> arch/x86/kvm/mmu/mmu.c | 267 +++++++++++++++++-
> arch/x86/kvm/x86.c | 6 +
> include/linux/kvm_host.h | 1 +
> virt/kvm/kvm_main.c | 2 +-
> 6 files changed, 293 insertions(+), 10 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 3f1cc5e317ed..bc3ad3d4df0b 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2387,8 +2387,7 @@
> the KVM_CLEAR_DIRTY ioctl, and only for the pages being
> cleared.
>
> - Eager page splitting currently only supports splitting
> - huge pages mapped by the TDP MMU.
> + Eager page splitting is only supported when kvm.tdp_mmu=Y.
>
> Default is Y (on).
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 9193a700fe2d..ea99e61cc556 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1265,6 +1265,28 @@ struct kvm_arch {
> * the global KVM_MAX_VCPU_IDS may lead to significant memory waste.
> */
> u32 max_vcpu_ids;
> +
> + /*
> + * Memory caches used to allocate shadow pages when performing eager
> + * page splitting. No need for a shadowed_info_cache since eager page
> + * splitting only allocates direct shadow pages.
> + *
> + * Protected by kvm->slots_lock.
> + */
> + struct kvm_mmu_memory_cache split_shadow_page_cache;
> + struct kvm_mmu_memory_cache split_page_header_cache;
> +
> + /*
> + * Memory cache used to allocate pte_list_desc structs while splitting
> + * huge pages. In the worst case, to split one huge page, 512
> + * pte_list_desc structs are needed to add each lower level leaf sptep
> + * to the rmap plus 1 to extend the parent_ptes rmap of the lower level
> + * page table.
> + *
> + * Protected by kvm->slots_lock.
> + */
> +#define SPLIT_DESC_CACHE_CAPACITY 513
> + struct kvm_mmu_memory_cache split_desc_cache;
> };
>
> struct kvm_vm_stat {
> @@ -1639,6 +1661,8 @@ void kvm_mmu_zap_all(struct kvm *kvm);
> void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm, u64 gen);
> void kvm_mmu_change_mmu_pages(struct kvm *kvm, unsigned long kvm_nr_mmu_pages);
>
> +void free_split_caches(struct kvm *kvm);
> +
> int load_pdptrs(struct kvm_vcpu *vcpu, unsigned long cr3);
>
> int emulator_write_phys(struct kvm_vcpu *vcpu, gpa_t gpa,
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 964a8fa63e1b..7c5eab61c4ea 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5966,6 +5966,15 @@ int kvm_mmu_init_vm(struct kvm *kvm)
> node->track_write = kvm_mmu_pte_write;
> node->track_flush_slot = kvm_mmu_invalidate_zap_pages_in_memslot;
> kvm_page_track_register_notifier(kvm, node);
> +
> + kvm->arch.split_page_header_cache.kmem_cache = mmu_page_header_cache;
> + kvm->arch.split_page_header_cache.gfp_zero = __GFP_ZERO;
> +
> + kvm->arch.split_shadow_page_cache.gfp_zero = __GFP_ZERO;
> +
> + kvm->arch.split_desc_cache.kmem_cache = pte_list_desc_cache;
> + kvm->arch.split_desc_cache.gfp_zero = __GFP_ZERO;
> +
> return 0;
> }
>
> @@ -6097,15 +6106,252 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm,
> kvm_arch_flush_remote_tlbs_memslot(kvm, memslot);
> }
>
> +void free_split_caches(struct kvm *kvm)
> +{
> + lockdep_assert_held(&kvm->slots_lock);
> +
> + kvm_mmu_free_memory_cache(&kvm->arch.split_desc_cache);
> + kvm_mmu_free_memory_cache(&kvm->arch.split_page_header_cache);
> + kvm_mmu_free_memory_cache(&kvm->arch.split_shadow_page_cache);
> +}
> +
> +static inline bool need_topup(struct kvm_mmu_memory_cache *cache, int min)
> +{
> + return kvm_mmu_memory_cache_nr_free_objects(cache) < min;
> +}
> +
> +static bool need_topup_split_caches_or_resched(struct kvm *kvm)
> +{
> + if (need_resched() || rwlock_needbreak(&kvm->mmu_lock))
> + return true;
> +
> + /*
> + * In the worst case, SPLIT_DESC_CACHE_CAPACITY descriptors are needed
> + * to split a single huge page. Calculating how many are actually needed
> + * is possible but not worth the complexity.
> + */
> + return need_topup(&kvm->arch.split_desc_cache, SPLIT_DESC_CACHE_CAPACITY) ||
> + need_topup(&kvm->arch.split_page_header_cache, 1) ||
> + need_topup(&kvm->arch.split_shadow_page_cache, 1);
> +}
> +
> +static int topup_split_caches(struct kvm *kvm)
> +{
> + int r;
> +
> + lockdep_assert_held(&kvm->slots_lock);
> +
> + r = __kvm_mmu_topup_memory_cache(&kvm->arch.split_desc_cache,
> + SPLIT_DESC_CACHE_CAPACITY,
> + SPLIT_DESC_CACHE_CAPACITY);
> + if (r)
> + return r;
> +
> + r = kvm_mmu_topup_memory_cache(&kvm->arch.split_page_header_cache, 1);
> + if (r)
> + return r;
> +
> + return kvm_mmu_topup_memory_cache(&kvm->arch.split_shadow_page_cache, 1);
> +}
> +
> +static struct kvm_mmu_page *nested_mmu_get_sp_for_split(struct kvm *kvm, u64 *huge_sptep)
> +{
> + struct kvm_mmu_page *huge_sp = sptep_to_sp(huge_sptep);
> + struct shadow_page_caches caches = {};
> + union kvm_mmu_page_role role;
> + unsigned int access;
> + gfn_t gfn;
> +
> + gfn = kvm_mmu_page_get_gfn(huge_sp, huge_sptep - huge_sp->spt);
> + access = kvm_mmu_page_get_access(huge_sp, huge_sptep - huge_sp->spt);
> +
> + /*
> + * Note, huge page splitting always uses direct shadow pages, regardless
> + * of whether the huge page itself is mapped by a direct or indirect
> + * shadow page, since the huge page region itself is being directly
> + * mapped with smaller pages.
> + */
> + role = kvm_mmu_child_role(huge_sptep, /*direct=*/true, access);
> +
> + /* Direct SPs do not require a shadowed_info_cache. */
> + caches.page_header_cache = &kvm->arch.split_page_header_cache;
> + caches.shadow_page_cache = &kvm->arch.split_shadow_page_cache;
> +
> + /* Safe to pass NULL for vCPU since requesting a direct SP. */
> + return __kvm_mmu_get_shadow_page(kvm, NULL, &caches, gfn, role);
> +}
> +
> +static void nested_mmu_split_huge_page(struct kvm *kvm,
> + const struct kvm_memory_slot *slot,
> + u64 *huge_sptep)
> +
> +{
> + struct kvm_mmu_memory_cache *cache = &kvm->arch.split_desc_cache;
> + u64 huge_spte = READ_ONCE(*huge_sptep);
> + struct kvm_mmu_page *sp;
> + bool flush = false;
> + u64 *sptep, spte;
> + gfn_t gfn;
> + int index;
> +
> + sp = nested_mmu_get_sp_for_split(kvm, huge_sptep);
> +
> + for (index = 0; index < PT64_ENT_PER_PAGE; index++) {
> + sptep = &sp->spt[index];
> + gfn = kvm_mmu_page_get_gfn(sp, index);
> +
> + /*
> + * The SP may already have populated SPTEs, e.g. if this huge
> + * page is aliased by multiple sptes with the same access
> + * permissions. These entries are guaranteed to map the same
> + * gfn-to-pfn translation since the SP is direct, so no need to
> + * modify them.
> + *
> + * However, if a given SPTE points to a lower level page table,
> + * that lower level page table may only be partially populated.
> + * Installing such SPTEs would effectively unmap a potion of the
> + * huge page. Unmapping guest memory always requires a TLB flush
> + * since a subsequent operation on the unmapped regions would
> + * fail to detect the need to flush.
> + */
> + if (is_shadow_present_pte(*sptep)) {
> + flush |= !is_last_spte(*sptep, sp->role.level);
> + continue;
> + }
> +
> + spte = make_huge_page_split_spte(huge_spte, sp->role, index);
> + mmu_spte_set(sptep, spte);
> + __rmap_add(kvm, cache, slot, sptep, gfn, sp->role.access);
> + }
> +
> + /*
> + * Replace the huge spte with a pointer to the populated lower level
> + * page table. If the lower-level page table indentically maps the huge
> + * page (i.e. no memory is unmapped), there's no need for a TLB flush.
> + * Otherwise, flush TLBs after dropping the huge page and before
> + * installing the shadow page table.
> + */
> + __drop_large_spte(kvm, huge_sptep, flush);
> + __link_shadow_page(cache, huge_sptep, sp);
> +}
> +
> +static int nested_mmu_try_split_huge_page(struct kvm *kvm,
> + const struct kvm_memory_slot *slot,
> + u64 *huge_sptep)
> +{
> + struct kvm_mmu_page *huge_sp = sptep_to_sp(huge_sptep);
> + int level, r = 0;
> + gfn_t gfn;
> + u64 spte;
> +
> + /* Grab information for the tracepoint before dropping the MMU lock. */
> + gfn = kvm_mmu_page_get_gfn(huge_sp, huge_sptep - huge_sp->spt);
> + level = huge_sp->role.level;
> + spte = *huge_sptep;
> +
> + if (kvm_mmu_available_pages(kvm) <= KVM_MIN_FREE_MMU_PAGES) {
> + r = -ENOSPC;
> + goto out;
> + }
> +
> + if (need_topup_split_caches_or_resched(kvm)) {
> + write_unlock(&kvm->mmu_lock);
> + cond_resched();
> + /*
> + * If the topup succeeds, return -EAGAIN to indicate that the
> + * rmap iterator should be restarted because the MMU lock was
> + * dropped.
> + */
> + r = topup_split_caches(kvm) ?: -EAGAIN;
> + write_lock(&kvm->mmu_lock);
> + goto out;
> + }
> +
> + nested_mmu_split_huge_page(kvm, slot, huge_sptep);
> +
> +out:
> + trace_kvm_mmu_split_huge_page(gfn, spte, level, r);
> + return r;
> +}
> +
> +static bool nested_mmu_try_split_huge_pages(struct kvm *kvm,
> + struct kvm_rmap_head *rmap_head,
> + const struct kvm_memory_slot *slot)
> +{
> + struct rmap_iterator iter;
> + struct kvm_mmu_page *sp;
> + u64 *huge_sptep;
> + int r;
> +
> +restart:
> + for_each_rmap_spte(rmap_head, &iter, huge_sptep) {
> + sp = sptep_to_sp(huge_sptep);
> +
> + /* TDP MMU is enabled, so rmap only contains nested MMU SPs. */
> + if (WARN_ON_ONCE(!sp->role.guest_mode))
> + continue;
> +
> + /* The rmaps should never contain non-leaf SPTEs. */
> + if (WARN_ON_ONCE(!is_large_pte(*huge_sptep)))
> + continue;
> +
> + /* SPs with level >PG_LEVEL_4K should never by unsync. */
> + if (WARN_ON_ONCE(sp->unsync))
> + continue;
> +
> + /* Don't bother splitting huge pages on invalid SPs. */
> + if (sp->role.invalid)
> + continue;
> +
> + r = nested_mmu_try_split_huge_page(kvm, slot, huge_sptep);
> +
> + /*
> + * The split succeeded or needs to be retried because the MMU
> + * lock was dropped. Either way, restart the iterator to get it
> + * back into a consistent state.
> + */
> + if (!r || r == -EAGAIN)
> + goto restart;
> +
> + /* The split failed and shouldn't be retried (e.g. -ENOMEM). */
> + break;
> + }
> +
> + return false;
> +}
> +
> +static void kvm_nested_mmu_try_split_huge_pages(struct kvm *kvm,
> + const struct kvm_memory_slot *slot,
> + gfn_t start, gfn_t end,
> + int target_level)
> +{
> + int level;
> +
> + /*
> + * Split huge pages starting with KVM_MAX_HUGEPAGE_LEVEL and working
> + * down to the target level. This ensures pages are recursively split
> + * all the way to the target level. There's no need to split pages
> + * already at the target level.
> + */
> + for (level = KVM_MAX_HUGEPAGE_LEVEL; level > target_level; level--) {
> + slot_handle_level_range(kvm, slot, nested_mmu_try_split_huge_pages,
> + level, level, start, end - 1, true, false);
> + }
> +}
> +
> /* Must be called with the mmu_lock held in write-mode. */
> void kvm_mmu_try_split_huge_pages(struct kvm *kvm,
> const struct kvm_memory_slot *memslot,
> u64 start, u64 end,
> int target_level)
> {
> - if (is_tdp_mmu_enabled(kvm))
> - kvm_tdp_mmu_try_split_huge_pages(kvm, memslot, start, end,
> - target_level, false);
> + if (!is_tdp_mmu_enabled(kvm))
> + return;
> +
> + if (kvm_memslots_have_rmaps(kvm))
> + kvm_nested_mmu_try_split_huge_pages(kvm, memslot, start, end, target_level);
> +
> + kvm_tdp_mmu_try_split_huge_pages(kvm, memslot, start, end, target_level, false);
>
> /*
> * A TLB flush is unnecessary at this point for the same resons as in
> @@ -6120,12 +6366,19 @@ void kvm_mmu_slot_try_split_huge_pages(struct kvm *kvm,
> u64 start = memslot->base_gfn;
> u64 end = start + memslot->npages;
>
> - if (is_tdp_mmu_enabled(kvm)) {
> - read_lock(&kvm->mmu_lock);
> - kvm_tdp_mmu_try_split_huge_pages(kvm, memslot, start, end, target_level, true);
> - read_unlock(&kvm->mmu_lock);
> + if (!is_tdp_mmu_enabled(kvm))
> + return;
> +
> + if (kvm_memslots_have_rmaps(kvm)) {
> + write_lock(&kvm->mmu_lock);
> + kvm_nested_mmu_try_split_huge_pages(kvm, memslot, start, end, target_level);
> + write_unlock(&kvm->mmu_lock);
> }
>
> + read_lock(&kvm->mmu_lock);
> + kvm_tdp_mmu_try_split_huge_pages(kvm, memslot, start, end, target_level, true);
> + read_unlock(&kvm->mmu_lock);
> +
> /*
> * No TLB flush is necessary here. KVM will flush TLBs after
> * write-protecting and/or clearing dirty on the newly split SPTEs to
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 04812eaaf61b..4fe018ddd1cd 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12197,6 +12197,12 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
> * page faults will create the large-page sptes.
> */
> kvm_mmu_zap_collapsible_sptes(kvm, new);
> +
> + /*
> + * Free any memory left behind by eager page splitting. Ignore
> + * the module parameter since userspace might have changed it.
> + */
> + free_split_caches(kvm);
> } else {
> /*
> * Initially-all-set does not require write protecting any page,
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index f94f72bbd2d3..17fc9247504d 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1336,6 +1336,7 @@ void kvm_flush_remote_tlbs(struct kvm *kvm);
>
> #ifdef KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE
> int kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min);
> +int __kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int capacity, int min);
If you end up with a v7, could you move this to the previous commit,
please. In that case this would include not making
__kvm_mmu_topup_memory_cache a static in the previous one as well.
Thanks,
Ricardo
> int kvm_mmu_memory_cache_nr_free_objects(struct kvm_mmu_memory_cache *mc);
> void kvm_mmu_free_memory_cache(struct kvm_mmu_memory_cache *mc);
> void *kvm_mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc);
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 5e2e75014256..b9573e958a03 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -369,7 +369,7 @@ static inline void *mmu_memory_cache_alloc_obj(struct kvm_mmu_memory_cache *mc,
> return (void *)__get_free_page(gfp_flags);
> }
>
> -static int __kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int capacity, int min)
> +int __kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int capacity, int min)
> {
> gfp_t gfp = GFP_KERNEL_ACCOUNT;
> void *obj;
> --
> 2.36.0.550.gb090851708-goog
>
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