KVM: arm64: Ignore EAGAIN for walks outside of a fault

The page table walkers are invoked outside fault handling paths, such as
write protecting a range of memory. EAGAIN is generally used by the
walkers to retry execution due to races on a particular PTE, like taking
an access fault on a PTE being invalidated from another thread.

This early return behavior is undesirable for walkers that operate
outside a fault handler. Suppress EAGAIN and continue the walk if
operating outside a fault handler.

Link: https://lore.kernel.org/r/20221202185156.696189-3-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index 192f33b..4cd6762 100644 (file)
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -193,12 +193,15 @@ typedef bool (*kvm_pgtable_force_pte_cb_t)(u64 addr, u64 end,
  *                                     children.
  * @KVM_PGTABLE_WALK_SHARED:           Indicates the page-tables may be shared
  *                                     with other software walkers.
+ * @KVM_PGTABLE_WALK_HANDLE_FAULT:     Indicates the page-table walk was
+ *                                     invoked from a fault handler.
  */
 enum kvm_pgtable_walk_flags {
        KVM_PGTABLE_WALK_LEAF                   = BIT(0),
        KVM_PGTABLE_WALK_TABLE_PRE              = BIT(1),
        KVM_PGTABLE_WALK_TABLE_POST             = BIT(2),
        KVM_PGTABLE_WALK_SHARED                 = BIT(3),
+       KVM_PGTABLE_WALK_HANDLE_FAULT           = BIT(4),
 };
 
 struct kvm_pgtable_visit_ctx {

diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index b11cf2c..9881821 100644 (file)
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -168,6 +168,25 @@ static int kvm_pgtable_visitor_cb(struct kvm_pgtable_walk_data *data,
        return walker->cb(ctx, visit);
 }
 
+static bool kvm_pgtable_walk_continue(const struct kvm_pgtable_walker *walker,
+                                     int r)
+{
+       /*
+        * Visitor callbacks return EAGAIN when the conditions that led to a
+        * fault are no longer reflected in the page tables due to a race to
+        * update a PTE. In the context of a fault handler this is interpreted
+        * as a signal to retry guest execution.
+        *
+        * Ignore the return code altogether for walkers outside a fault handler
+        * (e.g. write protecting a range of memory) and chug along with the
+        * page table walk.
+        */
+       if (r == -EAGAIN)
+               return !(walker->flags & KVM_PGTABLE_WALK_HANDLE_FAULT);
+
+       return !r;
+}
+
 static int __kvm_pgtable_walk(struct kvm_pgtable_walk_data *data,
                              struct kvm_pgtable_mm_ops *mm_ops, kvm_pteref_t pgtable, u32 level);
 
@@ -200,7 +219,7 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
                table = kvm_pte_table(ctx.old, level);
        }
 
-       if (ret)
+       if (!kvm_pgtable_walk_continue(data->walker, ret))
                goto out;
 
        if (!table) {
@@ -211,13 +230,16 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
 
        childp = (kvm_pteref_t)kvm_pte_follow(ctx.old, mm_ops);
        ret = __kvm_pgtable_walk(data, mm_ops, childp, level + 1);
-       if (ret)
+       if (!kvm_pgtable_walk_continue(data->walker, ret))
                goto out;
 
        if (ctx.flags & KVM_PGTABLE_WALK_TABLE_POST)
                ret = kvm_pgtable_visitor_cb(data, &ctx, KVM_PGTABLE_WALK_TABLE_POST);
 
 out:
+       if (kvm_pgtable_walk_continue(data->walker, ret))
+               return 0;
+
        return ret;
 }
 
@@ -1095,7 +1117,8 @@ kvm_pte_t kvm_pgtable_stage2_mkyoung(struct kvm_pgtable *pgt, u64 addr)
 {
        kvm_pte_t pte = 0;
        stage2_update_leaf_attrs(pgt, addr, 1, KVM_PTE_LEAF_ATTR_LO_S2_AF, 0,
-                                &pte, NULL, 0);
+                                &pte, NULL,
+                                KVM_PGTABLE_WALK_HANDLE_FAULT);
        dsb(ishst);
        return pte;
 }
@@ -1141,6 +1164,7 @@ int kvm_pgtable_stage2_relax_perms(struct kvm_pgtable *pgt, u64 addr,
                clr |= KVM_PTE_LEAF_ATTR_HI_S2_XN;
 
        ret = stage2_update_leaf_attrs(pgt, addr, 1, set, clr, NULL, &level,
+                                      KVM_PGTABLE_WALK_HANDLE_FAULT |
                                       KVM_PGTABLE_WALK_SHARED);
        if (!ret)
                kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, pgt->mmu, addr, level);

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 3ddee41..edfbe85 100644 (file)
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1381,7 +1381,9 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
        else
                ret = kvm_pgtable_stage2_map(pgt, fault_ipa, vma_pagesize,
                                             __pfn_to_phys(pfn), prot,
-                                            memcache, KVM_PGTABLE_WALK_SHARED);
+                                            memcache,
+                                            KVM_PGTABLE_WALK_HANDLE_FAULT |
+                                            KVM_PGTABLE_WALK_SHARED);
 
        /* Mark the page dirty only if the fault is handled successfully */
        if (writable && !ret) {