[PATCH mm-unstable v2 07/10] kvm/powerpc: add kvm_arch_test_clear_young()

Yu Zhao
2023-05-26T23:45:06Z

Implement kvm_arch_test_clear_young() to support the fast path in
mmu_notifier_ops->test_clear_young().

It focuses on a simple case, i.e., radix MMU sets the accessed bit in
KVM PTEs and VMs are not nested, where it can rely on RCU and
pte_xchg() to safely clear the accessed bit without taking
kvm->mmu_lock. Complex cases fall back to the existing slow path
where kvm->mmu_lock is then taken.

Signed-off-by: Yu Zhao <yuzhao@google.com>
---
arch/powerpc/include/asm/kvm_host.h | 8 ++++
arch/powerpc/include/asm/kvm_ppc.h | 1 +
arch/powerpc/kvm/book3s.c | 6 +++
arch/powerpc/kvm/book3s.h | 1 +
arch/powerpc/kvm/book3s_64_mmu_radix.c | 59 ++++++++++++++++++++++++++
arch/powerpc/kvm/book3s_hv.c | 5 +++
6 files changed, 80 insertions(+)

diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
index 14ee0dece853..75c260ea8a9e 100644
--- a/arch/powerpc/include/asm/kvm_host.h
+++ b/arch/powerpc/include/asm/kvm_host.h
@@ -883,4 +883,12 @@ static inline void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu) {}
static inline void kvm_arch_vcpu_blocking(struct kvm_vcpu *vcpu) {}
static inline void kvm_arch_vcpu_unblocking(struct kvm_vcpu *vcpu) {}

+#define kvm_arch_has_test_clear_young kvm_arch_has_test_clear_young
+static inline bool kvm_arch_has_test_clear_young(void)
+{
+ return IS_ENABLED(CONFIG_KVM_BOOK3S_HV_POSSIBLE) &&
+ cpu_has_feature(CPU_FTR_HVMODE) && cpu_has_feature(CPU_FTR_ARCH_300) &&
+ radix_enabled();
+}
+
#endif /* __POWERPC_KVM_HOST_H__ */
diff --git a/arch/powerpc/include/asm/kvm_ppc.h b/arch/powerpc/include/asm/kvm_ppc.h
index 79a9c0bb8bba..ff1af6a7b44f 100644
--- a/arch/powerpc/include/asm/kvm_ppc.h
+++ b/arch/powerpc/include/asm/kvm_ppc.h
@@ -287,6 +287,7 @@ struct kvmppc_ops {
bool (*unmap_gfn_range)(struct kvm *kvm, struct kvm_gfn_range *range);
bool (*age_gfn)(struct kvm *kvm, struct kvm_gfn_range *range);
bool (*test_age_gfn)(struct kvm *kvm, struct kvm_gfn_range *range);
+ bool (*test_clear_young)(struct kvm *kvm, struct kvm_gfn_range *range);
bool (*set_spte_gfn)(struct kvm *kvm, struct kvm_gfn_range *range);
void (*free_memslot)(struct kvm_memory_slot *slot);
int (*init_vm)(struct kvm *kvm);
diff --git a/arch/powerpc/kvm/book3s.c b/arch/powerpc/kvm/book3s.c
index 686d8d9eda3e..37bf40b0c4ff 100644
--- a/arch/powerpc/kvm/book3s.c
+++ b/arch/powerpc/kvm/book3s.c
@@ -899,6 +899,12 @@ bool kvm_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
return kvm->arch.kvm_ops->test_age_gfn(kvm, range);
}

+bool kvm_arch_test_clear_young(struct kvm *kvm, struct kvm_gfn_range *range)
+{
+ return !kvm->arch.kvm_ops->test_clear_young ||
+ kvm->arch.kvm_ops->test_clear_young(kvm, range);
+}
+
bool kvm_set_spte_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
{
return kvm->arch.kvm_ops->set_spte_gfn(kvm, range);
diff --git a/arch/powerpc/kvm/book3s.h b/arch/powerpc/kvm/book3s.h
index 58391b4b32ed..fa2659e21ccc 100644
--- a/arch/powerpc/kvm/book3s.h
+++ b/arch/powerpc/kvm/book3s.h
@@ -12,6 +12,7 @@ extern void kvmppc_core_flush_memslot_hv(struct kvm *kvm,
extern bool kvm_unmap_gfn_range_hv(struct kvm *kvm, struct kvm_gfn_range *range);
extern bool kvm_age_gfn_hv(struct kvm *kvm, struct kvm_gfn_range *range);
extern bool kvm_test_age_gfn_hv(struct kvm *kvm, struct kvm_gfn_range *range);
+extern bool kvm_test_clear_young_hv(struct kvm *kvm, struct kvm_gfn_range *range);
extern bool kvm_set_spte_gfn_hv(struct kvm *kvm, struct kvm_gfn_range *range);

extern int kvmppc_mmu_init_pr(struct kvm_vcpu *vcpu);
diff --git a/arch/powerpc/kvm/book3s_64_mmu_radix.c b/arch/powerpc/kvm/book3s_64_mmu_radix.c
index 3b65b3b11041..0a392e9a100a 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_radix.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_radix.c
@@ -1088,6 +1088,65 @@ bool kvm_test_age_radix(struct kvm *kvm, struct kvm_memory_slot *memslot,
return ref;
}

+bool kvm_test_clear_young_hv(struct kvm *kvm, struct kvm_gfn_range *range)
+{
+ bool err;
+ gfn_t gfn = range->start;
+
+ rcu_read_lock();
+
+ err = !kvm_is_radix(kvm);
+ if (err)
+ goto unlock;
+
+ /*
+ * Case 1: This function kvmppc_switch_mmu_to_hpt()
+ *
+ * rcu_read_lock()
+ * Test kvm_is_radix() kvm->arch.radix = 0
+ * Use kvm->arch.pgtable synchronize_rcu()
+ * rcu_read_unlock()
+ * kvmppc_free_radix()
+ *
+ *
+ * Case 2: This function kvmppc_switch_mmu_to_radix()
+ *
+ * kvmppc_init_vm_radix()
+ * smp_wmb()
+ * Test kvm_is_radix() kvm->arch.radix = 1
+ * smp_rmb()
+ * Use kvm->arch.pgtable
+ */
+ smp_rmb();
+
+ while (gfn < range->end) {
+ pte_t *ptep;
+ pte_t old, new;
+ unsigned int shift;
+
+ ptep = find_kvm_secondary_pte_unlocked(kvm, gfn * PAGE_SIZE, &shift);
+ if (!ptep)
+ goto next;
+
+ VM_WARN_ON_ONCE(!page_count(virt_to_page(ptep)));
+
+ old = READ_ONCE(*ptep);
+ if (!pte_present(old) || !pte_young(old))
+ goto next;
+
+ new = pte_mkold(old);
+
+ if (kvm_should_clear_young(range, gfn))
+ pte_xchg(ptep, old, new);
+next:
+ gfn += shift ? BIT(shift - PAGE_SHIFT) : 1;
+ }
+unlock:
+ rcu_read_unlock();
+
+ return err;
+}
+
/* Returns the number of PAGE_SIZE pages that are dirty */
static int kvm_radix_test_clear_dirty(struct kvm *kvm,
struct kvm_memory_slot *memslot, int pagenum)
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 130bafdb1430..20a81ec9fde8 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -5262,6 +5262,8 @@ int kvmppc_switch_mmu_to_hpt(struct kvm *kvm)
spin_lock(&kvm->mmu_lock);
kvm->arch.radix = 0;
spin_unlock(&kvm->mmu_lock);
+ /* see the comments in kvm_test_clear_young_hv() */
+ synchronize_rcu();
kvmppc_free_radix(kvm);

lpcr = LPCR_VPM1;
@@ -5286,6 +5288,8 @@ int kvmppc_switch_mmu_to_radix(struct kvm *kvm)
if (err)
return err;
kvmppc_rmap_reset(kvm);
+ /* see the comments in kvm_test_clear_young_hv() */
+ smp_wmb();
/* Mutual exclusion with kvm_unmap_gfn_range etc. */
spin_lock(&kvm->mmu_lock);
kvm->arch.radix = 1;
@@ -6185,6 +6189,7 @@ static struct kvmppc_ops kvm_ops_hv = {
.unmap_gfn_range = kvm_unmap_gfn_range_hv,
.age_gfn = kvm_age_gfn_hv,
.test_age_gfn = kvm_test_age_gfn_hv,
+ .test_clear_young = kvm_test_clear_young_hv,
.set_spte_gfn = kvm_set_spte_gfn_hv,
.free_memslot = kvmppc_core_free_memslot_hv,
.init_vm = kvmppc_core_init_vm_hv,
--
2.41.0.rc0.172.g3f132b7071-goog