[PATCH 11/14] KVM: arm64: Make changes block->table to leaf PTEs parallel-aware

[Oliver Upton <oliver.upton@linux.dev>]

In order to service stage-2 faults in parallel, stage-2 table walkers
must take exclusive ownership of the PTE being worked on. An additional
requirement of the architecture is that software must perform a
'break-before-make' operation when changing the block size used for
mapping memory.

Roll these two concepts together into helpers for performing a
'break-before-make' sequence. Use a special PTE value to indicate a PTE
has been locked by a software walker. Additionally, use an atomic
compare-exchange to 'break' the PTE when the stage-2 page tables are
possibly shared with another software walker. Elide the DSB + TLBI if
the evicted PTE was invalid (and thus not subject to break-before-make).

All of the atomics do nothing for now, as the stage-2 walker isn't fully
ready to perform parallel walks.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
 arch/arm64/kvm/hyp/pgtable.c | 87 +++++++++++++++++++++++++++++++++---
 1 file changed, 82 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 61a4437c8c16..71ae96608752 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -49,6 +49,12 @@
 #define KVM_INVALID_PTE_OWNER_MASK	GENMASK(9, 2)
 #define KVM_MAX_OWNER_ID		1
 
+/*
+ * Used to indicate a pte for which a 'break-before-make' sequence is in
+ * progress.
+ */
+#define KVM_INVALID_PTE_LOCKED		BIT(10)
+
 struct kvm_pgtable_walk_data {
 	struct kvm_pgtable		*pgt;
 	struct kvm_pgtable_walker	*walker;
@@ -586,6 +592,8 @@ struct stage2_map_data {
 
 	/* Force mappings to page granularity */
 	bool				force_pte;
+
+	bool				shared;
 };
 
 u64 kvm_get_vtcr(u64 mmfr0, u64 mmfr1, u32 phys_shift)
@@ -691,6 +699,11 @@ static bool stage2_pte_is_counted(kvm_pte_t pte)
 	return kvm_pte_valid(pte) || kvm_invalid_pte_owner(pte);
 }
 
+static bool stage2_pte_is_locked(kvm_pte_t pte)
+{
+	return !kvm_pte_valid(pte) && (pte & KVM_INVALID_PTE_LOCKED);
+}
+
 static bool stage2_try_set_pte(kvm_pte_t *ptep, kvm_pte_t old, kvm_pte_t new, bool shared)
 {
 	if (!shared) {
@@ -701,6 +714,69 @@ static bool stage2_try_set_pte(kvm_pte_t *ptep, kvm_pte_t old, kvm_pte_t new, bo
 	return cmpxchg(ptep, old, new) == old;
 }
 
+/**
+ * stage2_try_break_pte() - Invalidates a pte according to the
+ *			    'break-before-make' requirements of the
+ *			    architecture.
+ *
+ * @ptep: Pointer to the pte to break
+ * @old: The previously observed value of the pte
+ * @addr: IPA corresponding to the pte
+ * @level: Table level of the pte
+ * @shared: true if the stage-2 page tables could be shared by multiple software
+ *	    walkers
+ *
+ * Returns: true if the pte was successfully broken.
+ *
+ * If the removed pte was valid, performs the necessary serialization and TLB
+ * invalidation for the old value. For counted ptes, drops the reference count
+ * on the containing table page.
+ */
+static bool stage2_try_break_pte(kvm_pte_t *ptep, kvm_pte_t old, u64 addr, u32 level,
+				 struct stage2_map_data *data)
+{
+	struct kvm_pgtable_mm_ops *mm_ops = data->mm_ops;
+
+	if (stage2_pte_is_locked(old)) {
+		/*
+		 * Should never occur if this walker has exclusive access to the
+		 * page tables.
+		 */
+		WARN_ON(!data->shared);
+		return false;
+	}
+
+	if (!stage2_try_set_pte(ptep, old, KVM_INVALID_PTE_LOCKED, data->shared))
+		return false;
+
+	/*
+	 * Perform the appropriate TLB invalidation based on the evicted pte
+	 * value (if any).
+	 */
+	if (kvm_pte_table(old, level))
+		kvm_call_hyp(__kvm_tlb_flush_vmid, data->mmu);
+	else if (kvm_pte_valid(old))
+		kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, level);
+
+	if (stage2_pte_is_counted(old))
+		mm_ops->put_page(ptep);
+
+	return true;
+}
+
+static void stage2_make_pte(kvm_pte_t *ptep, kvm_pte_t old, kvm_pte_t new,
+			    struct stage2_map_data *data)
+{
+	struct kvm_pgtable_mm_ops *mm_ops = data->mm_ops;
+
+	WARN_ON(!stage2_pte_is_locked(*ptep));
+
+	if (stage2_pte_is_counted(new))
+		mm_ops->get_page(ptep);
+
+	smp_store_release(ptep, new);
+}
+
 static void stage2_put_pte(kvm_pte_t *ptep, struct kvm_s2_mmu *mmu, u64 addr,
 			   u32 level, struct kvm_pgtable_mm_ops *mm_ops)
 {
@@ -836,17 +912,18 @@ static int stage2_map_walk_leaf(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
 	if (!childp)
 		return -ENOMEM;
 
+	if (!stage2_try_break_pte(ptep, *old, addr, level, data)) {
+		mm_ops->put_page(childp);
+		return -EAGAIN;
+	}
+
 	/*
 	 * If we've run into an existing block mapping then replace it with
 	 * a table. Accesses beyond 'end' that fall within the new table
 	 * will be mapped lazily.
 	 */
-	if (stage2_pte_is_counted(pte))
-		stage2_put_pte(ptep, data->mmu, addr, level, mm_ops);
-
 	new = kvm_init_table_pte(childp, mm_ops);
-	mm_ops->get_page(ptep);
-	smp_store_release(ptep, new);
+	stage2_make_pte(ptep, *old, new, data);
 	*old = new;
 
 	return 0;
-- 
2.37.2.672.g94769d06f0-goog


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