[PATCH v3 2/5] RISC-V: KVM: Split huge pages when dirty logging is enabled
From: Wang Yechao
Date: Tue Jun 23 2026 - 08:17:23 EST
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.
The benefits of eager page splitting are the same as in x86 and arm64,
added with commit a3fe5dbda0a4 ("KVM: x86/mmu: Split huge pages mapped
by the TDP MMU when dirty logging is enabled") and commit e7bf7a490c68
("KVM: arm64: Split huge pages when dirty logging is enabled")
Signed-off-by: Wang Yechao <wang.yechao255@xxxxxxxxxx>
---
arch/riscv/include/asm/kvm_gstage.h | 6 +--
arch/riscv/kvm/gstage.c | 15 +++---
arch/riscv/kvm/mmu.c | 74 +++++++++++++++++++++++++++++
3 files changed, 86 insertions(+), 9 deletions(-)
diff --git a/arch/riscv/include/asm/kvm_gstage.h b/arch/riscv/include/asm/kvm_gstage.h
index 21e2019df0cf5..f726279780177 100644
--- a/arch/riscv/include/asm/kvm_gstage.h
+++ b/arch/riscv/include/asm/kvm_gstage.h
@@ -64,9 +64,9 @@ int kvm_riscv_gstage_map_page(struct kvm_gstage *gstage,
bool page_rdonly, bool page_exec,
struct kvm_gstage_mapping *out_map);
-int kvm_riscv_gstage_split_huge(struct kvm_gstage *gstage,
- struct kvm_mmu_memory_cache *pcache,
- gpa_t addr, u32 target_level, bool flush);
+bool kvm_riscv_gstage_split_huge(struct kvm_gstage *gstage,
+ struct kvm_mmu_memory_cache *pcache,
+ gpa_t addr, u32 target_level, bool flush);
enum kvm_riscv_gstage_op {
GSTAGE_OP_NOP = 0, /* Nothing */
diff --git a/arch/riscv/kvm/gstage.c b/arch/riscv/kvm/gstage.c
index c4c3b79567f10..1e980b4a08dc5 100644
--- a/arch/riscv/kvm/gstage.c
+++ b/arch/riscv/kvm/gstage.c
@@ -303,19 +303,20 @@ static inline unsigned long make_child_pte(unsigned long huge_pte, int index,
return child_pte;
}
-int kvm_riscv_gstage_split_huge(struct kvm_gstage *gstage,
- struct kvm_mmu_memory_cache *pcache,
- gpa_t addr, u32 target_level, bool flush)
+bool kvm_riscv_gstage_split_huge(struct kvm_gstage *gstage,
+ struct kvm_mmu_memory_cache *pcache,
+ gpa_t addr, u32 target_level, bool flush)
{
u32 current_level = gstage->pgd_levels - 1;
pte_t *next_ptep = (pte_t *)gstage->pgd;
unsigned long huge_pte, child_pte;
unsigned long child_page_size;
+ bool need_flush = false;
pte_t *ptep;
int i, ret;
if (!pcache)
- return -ENOMEM;
+ return false;
while(current_level > target_level) {
ptep = (pte_t *)&next_ptep[gstage_pte_index(gstage, addr, current_level)];
@@ -337,7 +338,7 @@ int kvm_riscv_gstage_split_huge(struct kvm_gstage *gstage,
next_ptep = kvm_mmu_memory_cache_alloc(pcache);
if (!next_ptep)
- return -ENOMEM;
+ return need_flush;
for (i = 0; i < PTRS_PER_PTE; i++) {
child_pte = make_child_pte(huge_pte, i, child_page_size);
@@ -349,11 +350,13 @@ int kvm_riscv_gstage_split_huge(struct kvm_gstage *gstage,
if (flush)
gstage_tlb_flush(gstage, current_level, addr);
+ else
+ need_flush = true;
current_level--;
}
- return 0;
+ return need_flush;
}
bool kvm_riscv_gstage_op_pte(struct kvm_gstage *gstage, gpa_t addr,
diff --git a/arch/riscv/kvm/mmu.c b/arch/riscv/kvm/mmu.c
index d69fba7d8e9a6..479f7e9bb8f81 100644
--- a/arch/riscv/kvm/mmu.c
+++ b/arch/riscv/kvm/mmu.c
@@ -97,6 +97,60 @@ void kvm_riscv_mmu_iounmap(struct kvm *kvm, gpa_t gpa, unsigned long size)
size >> PAGE_SHIFT);
}
+static bool need_topup_split_caches_or_resched(struct kvm_mmu_memory_cache *cache,
+ int count)
+{
+ if (need_resched())
+ return true;
+
+ return kvm_mmu_memory_cache_nr_free_objects(cache) < count;
+}
+
+static bool mmu_split_huge_pages(struct kvm_gstage *gstage,
+ phys_addr_t start, phys_addr_t end)
+{
+ struct kvm *kvm = gstage->kvm;
+ struct kvm_mmu_memory_cache *pcache = &kvm->arch.pgd_split_page_cache;
+ phys_addr_t addr = ALIGN(start, PMD_SIZE);
+ phys_addr_t last_flush_gfn = addr >> PAGE_SHIFT;
+ int count = gstage->pgd_levels;
+ bool flush = false;
+ int ret;
+
+ lockdep_assert_held_write(&kvm->mmu_lock);
+
+ while (addr < end) {
+ if (need_topup_split_caches_or_resched(pcache, count)) {
+ if (flush) {
+ kvm_flush_remote_tlbs_range(kvm, last_flush_gfn,
+ (addr >> PAGE_SHIFT) - last_flush_gfn);
+ last_flush_gfn = addr >> PAGE_SHIFT;
+ flush = false;
+ }
+
+ write_unlock(&kvm->mmu_lock);
+ cond_resched();
+
+ ret = kvm_mmu_topup_memory_cache(pcache, count);
+ if (ret) {
+ kvm_err("Failed to toup split page cache\n");
+ write_lock(&kvm->mmu_lock);
+ return flush;
+ }
+ write_lock(&kvm->mmu_lock);
+ }
+
+ if (!kvm->arch.pgd)
+ return flush;
+
+ flush |= kvm_riscv_gstage_split_huge(gstage, pcache, addr, 0, false);
+
+ addr += PMD_SIZE;
+ }
+
+ return flush;
+}
+
void kvm_arch_mmu_enable_log_dirty_pt_masked(struct kvm *kvm,
struct kvm_memory_slot *slot,
gfn_t gfn_offset,
@@ -151,6 +205,25 @@ void kvm_arch_flush_shadow_memslot(struct kvm *kvm,
size >> PAGE_SHIFT);
}
+static void 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 = memslot->base_gfn << PAGE_SHIFT;
+ phys_addr_t end = (memslot->base_gfn + memslot->npages) << PAGE_SHIFT;
+ struct kvm_gstage gstage;
+ bool flush;
+
+ kvm_riscv_gstage_init(&gstage, kvm);
+
+ write_lock(&kvm->mmu_lock);
+ flush = mmu_split_huge_pages(&gstage, start, end);
+ write_unlock(&kvm->mmu_lock);
+
+ if (flush)
+ kvm_flush_remote_tlbs_memslot(kvm, memslot);
+}
+
void kvm_arch_commit_memory_region(struct kvm *kvm,
struct kvm_memory_slot *old,
const struct kvm_memory_slot *new,
@@ -164,6 +237,7 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
if (kvm_dirty_log_manual_protect_and_init_set(kvm))
return;
mmu_wp_memory_region(kvm, new->id);
+ mmu_split_memory_region(kvm, new->id);
}
}
--
2.43.5