KVM: arm64: Split huge pages when dirty logging is enabled

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.

Eager page splitting only takes effect when the huge page mapping has
been existing in the stage-2 page table. Otherwise, the huge page will
be mapped to multiple non-huge pages on page fault.

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 <ricarkol@google.com>
Reviewed-by: Shaoqin Huang <shahuang@redhat.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Link: https://lore.kernel.org/r/20230426172330.1439644-10-ricarkol@google.com
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index d3fb35a002f96cbfaf3356d20a9a61e764195054..a36a01426b59a3f473678843471b2f2ce95c53e4 100644 (file)
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -31,14 +31,21 @@ static phys_addr_t __ro_after_init hyp_idmap_vector;
 
 static unsigned long __ro_after_init io_map_base;
 
-static phys_addr_t stage2_range_addr_end(phys_addr_t addr, phys_addr_t end)
+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,
@@ -75,6 +82,79 @@ static int stage2_apply_range(struct kvm_s2_mmu *mmu, phys_addr_t addr,
 #define stage2_apply_range_resched(mmu, addr, end, fn)                 \
        stage2_apply_range(mmu, addr, end, fn, true)
 
+/*
+ * Get the maximum number of page-tables pages needed to split a range
+ * of blocks into PAGE_SIZE PTEs. It assumes the range is already
+ * mapped at level 2, or at level 1 if allowed.
+ */
+static int kvm_mmu_split_nr_page_tables(u64 range)
+{
+       int n = 0;
+
+       if (KVM_PGTABLE_MIN_BLOCK_LEVEL < 2)
+               n += DIV_ROUND_UP_ULL(range, PUD_SIZE);
+       n += DIV_ROUND_UP_ULL(range, PMD_SIZE);
+       return n;
+}
+
+static bool need_split_memcache_topup_or_resched(struct kvm *kvm)
+{
+       struct kvm_mmu_memory_cache *cache;
+       u64 chunk_size, min;
+
+       if (need_resched() || rwlock_needbreak(&kvm->mmu_lock))
+               return true;
+
+       chunk_size = kvm->arch.mmu.split_page_chunk_size;
+       min = kvm_mmu_split_nr_page_tables(chunk_size);
+       cache = &kvm->arch.mmu.split_page_cache;
+       return kvm_mmu_memory_cache_nr_free_objects(cache) < min;
+}
+
+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, cache_capacity;
+       u64 next, chunk_size;
+
+       lockdep_assert_held_write(&kvm->mmu_lock);
+
+       chunk_size = kvm->arch.mmu.split_page_chunk_size;
+       cache_capacity = kvm_mmu_split_nr_page_tables(chunk_size);
+
+       if (chunk_size == 0)
+               return 0;
+
+       cache = &kvm->arch.mmu.split_page_cache;
+
+       do {
+               if (need_split_memcache_topup_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, chunk_size);
+               ret = kvm_pgtable_stage2_split(pgt, addr, next - addr, cache);
+               if (ret)
+                       break;
+       } while (addr = next, addr != end);
+
+       return ret;
+}
+
 static bool memslot_is_logging(struct kvm_memory_slot *memslot)
 {
        return memslot->dirty_bitmap && !(memslot->flags & KVM_MEM_READONLY);
@@ -793,6 +873,7 @@ out_free_pgtable:
 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,
@@ -1019,6 +1100,34 @@ 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;
+       struct kvm_memory_slot *memslot;
+       phys_addr_t start, end;
+
+       lockdep_assert_held(&kvm->slots_lock);
+
+       slots = kvm_memslots(kvm);
+       memslot = id_to_memslot(slots, slot);
+
+       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.
@@ -1812,8 +1921,8 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
                        return;
 
                /*
-                * Pages are write-protected on either of these two
-                * cases:
+                * Huge and normal pages are write-protected and split
+                * on either of these two cases:
                 *
                 * 1. with initial-all-set: gradually with CLEAR ioctls,
                 */
@@ -1825,6 +1934,16 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
                 *    enabling dirty logging.
                 */
                kvm_mmu_wp_memory_region(kvm, new->id);
+               kvm_mmu_split_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);
        }
 }