[PATCH v3 12/15] KVM: arm64: Make block->table PTE changes parallel-aware

Oliver Upton
2022-10-27T22:23:10Z

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 | 82 +++++++++++++++++++++++++++++++++---
1 file changed, 76 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 4c579b3beabf..1df858c21b2e 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_walker *walker;

@@ -674,6 +680,11 @@ static bool stage2_pte_is_counted(kvm_pte_t pte)
return !!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(const struct kvm_pgtable_visit_ctx *ctx, kvm_pte_t new)
{
if (!kvm_pgtable_walk_shared(ctx)) {
@@ -684,6 +695,64 @@ static bool stage2_try_set_pte(const struct kvm_pgtable_visit_ctx *ctx, kvm_pte_
return cmpxchg(ctx->ptep, ctx->old, new) == ctx->old;
}

+/**
+ * stage2_try_break_pte() - Invalidates a pte according to the
+ * 'break-before-make' requirements of the
+ * architecture.
+ *
+ * @ctx: context of the visited pte.
+ * @data: stage-2 map data
+ *
+ * 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(const struct kvm_pgtable_visit_ctx *ctx,
+ struct stage2_map_data *data)
+{
+ struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops;
+
+ if (stage2_pte_is_locked(ctx->old)) {
+ /*
+ * Should never occur if this walker has exclusive access to the
+ * page tables.
+ */
+ WARN_ON(!kvm_pgtable_walk_shared(ctx));
+ return false;
+ }
+
+ if (!stage2_try_set_pte(ctx, KVM_INVALID_PTE_LOCKED))
+ return false;
+
+ /*
+ * Perform the appropriate TLB invalidation based on the evicted pte
+ * value (if any).
+ */
+ if (kvm_pte_table(ctx->old, ctx->level))
+ kvm_call_hyp(__kvm_tlb_flush_vmid, data->mmu);
+ else if (kvm_pte_valid(ctx->old))
+ kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, ctx->addr, ctx->level);
+
+ if (stage2_pte_is_counted(ctx->old))
+ mm_ops->put_page(ctx->ptep);
+
+ return true;
+}
+
+static void stage2_make_pte(const struct kvm_pgtable_visit_ctx *ctx, kvm_pte_t new)
+{
+ struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops;
+
+ WARN_ON(!stage2_pte_is_locked(*ctx->ptep));
+
+ if (stage2_pte_is_counted(new))
+ mm_ops->get_page(ctx->ptep);
+
+ smp_store_release(ctx->ptep, new);
+}
+
static void stage2_put_pte(const struct kvm_pgtable_visit_ctx *ctx, struct kvm_s2_mmu *mmu,
struct kvm_pgtable_mm_ops *mm_ops)
{
@@ -795,7 +864,7 @@ static int stage2_map_walk_leaf(const struct kvm_pgtable_visit_ctx *ctx,
struct stage2_map_data *data)
{
struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops;
- kvm_pte_t *childp;
+ kvm_pte_t *childp, new;
int ret;

ret = stage2_map_walker_try_leaf(ctx, data);
@@ -812,17 +881,18 @@ static int stage2_map_walk_leaf(const struct kvm_pgtable_visit_ctx *ctx,
if (!childp)
return -ENOMEM;

+ if (!stage2_try_break_pte(ctx, 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(ctx->old))
- stage2_put_pte(ctx, data->mmu, mm_ops);
-
new = kvm_init_table_pte(childp, mm_ops);
- mm_ops->get_page(ctx->ptep);
- smp_store_release(ctx->ptep, new);
+ stage2_make_pte(ctx, new);

return 0;
}
--
2.38.1.273.g43a17bfeac-goog