[PATCH mm-unstable v2 04/10] kvm/arm64: make stage2 page tables RCU safe

Yu Zhao
2023-05-26T23:44:55Z

Stage2 page tables are currently not RCU safe against unmapping or VM
destruction. The previous mmu_notifier_ops members rely on
kvm->mmu_lock to synchronize with those operations.

However, the new mmu_notifier_ops member test_clear_young() provides
a fast path that does not take kvm->mmu_lock. To implement
kvm_arch_test_clear_young() for that path, unmapped page tables need
to be freed by RCU and kvm_free_stage2_pgd() needs to be after
mmu_notifier_unregister().

Remapping, specifically stage2_free_removed_table(), is already RCU
safe.

Signed-off-by: Yu Zhao <yuzhao@google.com>
---
arch/arm64/include/asm/kvm_pgtable.h | 2 ++
arch/arm64/kvm/arm.c | 1 +
arch/arm64/kvm/hyp/pgtable.c | 8 ++++++--
arch/arm64/kvm/mmu.c | 17 ++++++++++++++++-
4 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index ff520598b62c..5cab52e3a35f 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -153,6 +153,7 @@ static inline bool kvm_level_supports_block_mapping(u32 level)
* @put_page: Decrement the refcount on a page. When the
* refcount reaches 0 the page is automatically
* freed.
+ * @put_page_rcu: RCU variant of the above.
* @page_count: Return the refcount of a page.
* @phys_to_virt: Convert a physical address into a virtual
* address mapped in the current context.
@@ -170,6 +171,7 @@ struct kvm_pgtable_mm_ops {
void (*free_removed_table)(void *addr, u32 level);
void (*get_page)(void *addr);
void (*put_page)(void *addr);
+ void (*put_page_rcu)(void *addr);
int (*page_count)(void *addr);
void* (*phys_to_virt)(phys_addr_t phys);
phys_addr_t (*virt_to_phys)(void *addr);
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 14391826241c..ee93271035d9 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -191,6 +191,7 @@ vm_fault_t kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
*/
void kvm_arch_destroy_vm(struct kvm *kvm)
{
+ kvm_free_stage2_pgd(&kvm->arch.mmu);
bitmap_free(kvm->arch.pmu_filter);
free_cpumask_var(kvm->arch.supported_cpus);

diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 24678ccba76a..dbace4c6a841 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -988,8 +988,12 @@ static int stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx,
mm_ops->dcache_clean_inval_poc(kvm_pte_follow(ctx->old, mm_ops),
kvm_granule_size(ctx->level));

- if (childp)
- mm_ops->put_page(childp);
+ if (childp) {
+ if (mm_ops->put_page_rcu)
+ mm_ops->put_page_rcu(childp);
+ else
+ mm_ops->put_page(childp);
+ }

return 0;
}
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 3b9d4d24c361..c3b3e2afe26f 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -172,6 +172,21 @@ static int kvm_host_page_count(void *addr)
return page_count(virt_to_page(addr));
}

+static void kvm_s2_rcu_put_page(struct rcu_head *head)
+{
+ put_page(container_of(head, struct page, rcu_head));
+}
+
+static void kvm_s2_put_page_rcu(void *addr)
+{
+ struct page *page = virt_to_page(addr);
+
+ if (kvm_host_page_count(addr) == 1)
+ kvm_account_pgtable_pages(addr, -1);
+
+ call_rcu(&page->rcu_head, kvm_s2_rcu_put_page);
+}
+
static phys_addr_t kvm_host_pa(void *addr)
{
return __pa(addr);
@@ -704,6 +719,7 @@ static struct kvm_pgtable_mm_ops kvm_s2_mm_ops = {
.free_removed_table = stage2_free_removed_table,
.get_page = kvm_host_get_page,
.put_page = kvm_s2_put_page,
+ .put_page_rcu = kvm_s2_put_page_rcu,
.page_count = kvm_host_page_count,
.phys_to_virt = kvm_host_va,
.virt_to_phys = kvm_host_pa,
@@ -1877,7 +1893,6 @@ void kvm_arch_memslots_updated(struct kvm *kvm, u64 gen)

void kvm_arch_flush_shadow_all(struct kvm *kvm)
{
- kvm_free_stage2_pgd(&kvm->arch.mmu);
}

void kvm_arch_flush_shadow_memslot(struct kvm *kvm,
--
2.41.0.rc0.172.g3f132b7071-goog