[PATCH v5 21/21] KVM: x86/mmu: Extend Eager Page Splitting to nested MMUs

David Matlack
2022-05-13T20:29:33Z

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@google.com>
[ 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@google.com>
---
.../admin-guide/kernel-parameters.txt | 3 +-
arch/x86/include/asm/kvm_host.h | 24 ++
arch/x86/kvm/mmu/mmu.c | 269 +++++++++++++++++-
arch/x86/kvm/x86.c | 6 +
4 files changed, 293 insertions(+), 9 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 dad7e19ef8ed..3d3ec5e66195 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5981,6 +5981,18 @@ 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.capacity = KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE;
+ 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.capacity = KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE;
+ kvm->arch.split_shadow_page_cache.gfp_zero = __GFP_ZERO;
+
+ kvm->arch.split_desc_cache.capacity = SPLIT_DESC_CACHE_CAPACITY;
+ kvm->arch.split_desc_cache.kmem_cache = pte_list_desc_cache;
+ kvm->arch.split_desc_cache.gfp_zero = __GFP_ZERO;
+
return 0;
}

@@ -6112,15 +6124,251 @@ 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);
+ 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
@@ -6135,12 +6383,19 @@ 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,
--
2.36.0.550.gb090851708-goog