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

Oliver Upton
2022-08-30T19:51:31Z

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