Split huge pages eagerly when enabling dirty logging. The goal is to
avoid doing it while faulting on write-protected pages, which negatively
impacts guest performance.
A memslot marked for dirty logging is split in 1GB pieces at a time.
This is in order to release the mmu_lock and give other kernel threads
the opportunity to run, and also in order to allocate enough pages to
split a 1GB range worth of huge pages (or a single 1GB huge page). Note
that these page allocations can fail, so eager page splitting is
best-effort. This is not a correctness issue though, as huge pages can
still be split on write-faults.
The benefits of eager page splitting are the same as in x86, added with
commit a3fe5dbda0a4 ("KVM: x86/mmu: Split huge pages mapped by the TDP MMU
when dirty logging is enabled"). For example, when running
dirty_log_perf_test with 64 virtual CPUs (Ampere Altra), 1GB per vCPU, 50%
reads, and 2MB HugeTLB memory, the time it takes vCPUs to access all of
their memory after dirty logging is enabled decreased by 44% from 2.58s to
1.42s.
Signed-off-by: Ricardo Koller <ricar...@google.com>
---
arch/arm64/include/asm/kvm_host.h | 30 ++++++++
arch/arm64/kvm/mmu.c | 110 +++++++++++++++++++++++++++++-
2 files changed, 138 insertions(+), 2 deletions(-)
diff --git a/arch/arm64/include/asm/kvm_host.h
b/arch/arm64/include/asm/kvm_host.h
index 63307e7dc9c5..d43f133518cf 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -153,6 +153,36 @@ struct kvm_s2_mmu {
/* The last vcpu id that ran on each physical CPU */
int __percpu *last_vcpu_ran;
+ /*
+ * Memory cache used to split EAGER_PAGE_SPLIT_CHUNK_SIZE worth of huge
+ * pages. It is used to allocate stage2 page tables while splitting
+ * huge pages. Its capacity should be EAGER_PAGE_SPLIT_CACHE_CAPACITY.
+ * Note that the choice of EAGER_PAGE_SPLIT_CHUNK_SIZE influences both
+ * the capacity of the split page cache (CACHE_CAPACITY), 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 larger than
+ * all the available huge-page sizes, and be a multiple of all the
+ * other ones; for example, 1GB when all the available huge-page sizes
+ * are (1GB, 2MB, 32MB, 512MB).
+ *
+ * CACHE_CAPACITY should have enough pages to cover CHUNK_SIZE; for
+ * example, 1GB requires the following number of PAGE_SIZE-pages:
+ * - 512 when using 2MB hugepages with 4KB granules (1GB / 2MB).
+ * - 513 when using 1GB hugepages with 4KB granules (1 + (1GB / 2MB)).
+ * - 32 when using 32MB hugepages with 16KB granule (1GB / 32MB).
+ * - 2 when using 512MB hugepages with 64KB granules (1GB / 512MB).
+ * CACHE_CAPACITY below assumes the worst case: 1GB hugepages with 4KB
+ * granules.
+ *
+ * Protected by kvm->slots_lock.
+ */
+#define EAGER_PAGE_SPLIT_CHUNK_SIZE SZ_1G
+#define EAGER_PAGE_SPLIT_CACHE_CAPACITY
\
+ (DIV_ROUND_UP_ULL(EAGER_PAGE_SPLIT_CHUNK_SIZE, SZ_1G) + \
+ DIV_ROUND_UP_ULL(EAGER_PAGE_SPLIT_CHUNK_SIZE, SZ_2M))
+ struct kvm_mmu_memory_cache split_page_cache;
+
struct kvm_arch *arch;
};
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 94865c5ce181..f2753d9deb19 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -31,14 +31,24 @@ static phys_addr_t hyp_idmap_vector;
static unsigned long io_map_base;
-static phys_addr_t stage2_range_addr_end(phys_addr_t addr, phys_addr_t end)
+bool __read_mostly eager_page_split = true;
+module_param(eager_page_split, bool, 0644);
+
+static phys_addr_t __stage2_range_addr_end(phys_addr_t addr, phys_addr_t end,
+ phys_addr_t size)
{
- phys_addr_t size = kvm_granule_size(KVM_PGTABLE_MIN_BLOCK_LEVEL);
phys_addr_t boundary = ALIGN_DOWN(addr + size, size);
return (boundary - 1 < end - 1) ? boundary : end;
}
+static phys_addr_t stage2_range_addr_end(phys_addr_t addr, phys_addr_t end)
+{
+ phys_addr_t size = kvm_granule_size(KVM_PGTABLE_MIN_BLOCK_LEVEL);
+
+ return __stage2_range_addr_end(addr, end, size);
+}
+
/*
* Release kvm_mmu_lock periodically if the memory region is large. Otherwise,
* we may see kernel panics with CONFIG_DETECT_HUNG_TASK,
@@ -71,6 +81,64 @@ static int stage2_apply_range(struct kvm *kvm, phys_addr_t
addr,
return ret;
}
+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_page_cache_or_resched(struct kvm *kvm)
+{
+ struct kvm_mmu_memory_cache *cache;
+
+ if (need_resched() || rwlock_needbreak(&kvm->mmu_lock))
+ return true;
+
+ cache = &kvm->arch.mmu.split_page_cache;
+ return need_topup(cache, EAGER_PAGE_SPLIT_CACHE_CAPACITY);
+}
+
+static int kvm_mmu_split_huge_pages(struct kvm *kvm, phys_addr_t addr,
+ phys_addr_t end)
+{
+ struct kvm_mmu_memory_cache *cache;
+ struct kvm_pgtable *pgt;
+ int ret;
+ u64 next;
+ int cache_capacity = EAGER_PAGE_SPLIT_CACHE_CAPACITY;
+
+ lockdep_assert_held_write(&kvm->mmu_lock);
+
+ lockdep_assert_held(&kvm->slots_lock);
+
+ cache = &kvm->arch.mmu.split_page_cache;
+
+ do {
+ if (need_topup_split_page_cache_or_resched(kvm)) {
+ write_unlock(&kvm->mmu_lock);
+ cond_resched();
+ /* Eager page splitting is best-effort. */
+ ret = __kvm_mmu_topup_memory_cache(cache,
+ cache_capacity,
+ cache_capacity);
+ write_lock(&kvm->mmu_lock);
+ if (ret)
+ break;
+ }
+
+ pgt = kvm->arch.mmu.pgt;
+ if (!pgt)
+ return -EINVAL;
+
+ next = __stage2_range_addr_end(addr, end,
+ EAGER_PAGE_SPLIT_CHUNK_SIZE);
+ ret = kvm_pgtable_stage2_split(pgt, addr, next - addr, cache);
+ if (ret)
+ break;
+ } while (addr = next, addr != end);
+
+ return ret;
+}
+
#define stage2_apply_range_resched(kvm, addr, end, fn) \
stage2_apply_range(kvm, addr, end, fn, true)
@@ -755,6 +823,8 @@ 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->pgt = pgt;
mmu->pgd_phys = __pa(pgt->pgd);
return 0;
@@ -769,6 +839,7 @@ int kvm_init_stage2_mmu(struct kvm *kvm, struct kvm_s2_mmu
*mmu, unsigned long t
void kvm_uninit_stage2_mmu(struct kvm *kvm)
{
kvm_free_stage2_pgd(&kvm->arch.mmu);
+ kvm_mmu_free_memory_cache(&kvm->arch.mmu.split_page_cache);
}
static void stage2_unmap_memslot(struct kvm *kvm,
@@ -996,6 +1067,29 @@ static void kvm_mmu_write_protect_pt_masked(struct kvm
*kvm,
stage2_wp_range(&kvm->arch.mmu, start, end);
}
+/**
+ * kvm_mmu_split_memory_region() - split the stage 2 blocks into PAGE_SIZE
+ * pages for memory slot
+ * @kvm: The KVM pointer
+ * @slot: The memory slot to split
+ *
+ * Acquires kvm->mmu_lock. Called with kvm->slots_lock mutex acquired,
+ * serializing operations for VM memory regions.
+ */
+static void kvm_mmu_split_memory_region(struct kvm *kvm, int slot)
+{
+ struct kvm_memslots *slots = kvm_memslots(kvm);
+ struct kvm_memory_slot *memslot = id_to_memslot(slots, slot);
+ phys_addr_t start, end;
+
+ start = memslot->base_gfn << PAGE_SHIFT;
+ end = (memslot->base_gfn + memslot->npages) << PAGE_SHIFT;
+
+ write_lock(&kvm->mmu_lock);
+ kvm_mmu_split_huge_pages(kvm, start, end);
+ write_unlock(&kvm->mmu_lock);
+}
+
/*
* kvm_arch_mmu_enable_log_dirty_pt_masked - enable dirty logging for selected
* dirty pages.
@@ -1795,7 +1889,19 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
if (kvm_dirty_log_manual_protect_and_init_set(kvm))
return;
+ if (READ_ONCE(eager_page_split))
+ kvm_mmu_split_memory_region(kvm, new->id);
+
kvm_mmu_wp_memory_region(kvm, new->id);
+ } else {
+ /*
+ * Free any leftovers from the eager page splitting cache. Do
+ * this when deleting, moving, disabling dirty logging, or
+ * creating the memslot (a nop). Doing it for deletes makes
+ * sure we don't leak memory, and there's no need to keep the
+ * cache around for any of the other cases.
+ */
+ kvm_mmu_free_memory_cache(&kvm->arch.mmu.split_page_cache);
}
}
--
2.38.1.431.g37b22c650d-goog
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