On Mon, Nov 7, 2022 at 1:58 PM Oliver Upton <oliver.upton@xxxxxxxxx> wrote:
>
> The stage2 attr walker is already used for parallel walks. Since commit
> f783ef1c0e82 ("KVM: arm64: Add fast path to handle permission relaxation
> during dirty logging"), KVM acquires the read lock when
> write-unprotecting a PTE. However, the walker only uses a simple store
> to update the PTE. This is safe as the only possible race is with
> hardware updates to the access flag, which is benign.
>
> However, a subsequent change to KVM will allow more changes to the stage
> 2 page tables to be done in parallel. Prepare the stage 2 attribute
> walker by performing atomic updates to the PTE when walking in parallel.
>
> Signed-off-by: Oliver Upton <oliver.upton@xxxxxxxxx>
> ---
> arch/arm64/kvm/hyp/pgtable.c | 31 ++++++++++++++++++++++---------
> 1 file changed, 22 insertions(+), 9 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index d8d963521d4e..a34e2050f931 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -185,7 +185,7 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
> kvm_pteref_t pteref, u32 level)
> {
> enum kvm_pgtable_walk_flags flags = data->walker->flags;
> - kvm_pte_t *ptep = kvm_dereference_pteref(pteref, false);
> + kvm_pte_t *ptep = kvm_dereference_pteref(pteref, flags & KVM_PGTABLE_WALK_SHARED);
> struct kvm_pgtable_visit_ctx ctx = {
> .ptep = ptep,
> .old = READ_ONCE(*ptep),
> @@ -675,6 +675,16 @@ static bool stage2_pte_is_counted(kvm_pte_t pte)
> return !!pte;
> }
>
> +static bool stage2_try_set_pte(const struct kvm_pgtable_visit_ctx *ctx, kvm_pte_t new)
> +{
> + if (!kvm_pgtable_walk_shared(ctx)) {
> + WRITE_ONCE(*ctx->ptep, new);
> + return true;
> + }
> +
> + return cmpxchg(ctx->ptep, ctx->old, new) == ctx->old;
> +}
> +
> static void stage2_put_pte(const struct kvm_pgtable_visit_ctx *ctx, struct kvm_s2_mmu *mmu,
> struct kvm_pgtable_mm_ops *mm_ops)
> {
> @@ -986,7 +996,9 @@ static int stage2_attr_walker(const struct kvm_pgtable_visit_ctx *ctx,
> stage2_pte_executable(pte) && !stage2_pte_executable(ctx->old))
> mm_ops->icache_inval_pou(kvm_pte_follow(pte, mm_ops),
> kvm_granule_size(ctx->level));
> - WRITE_ONCE(*ctx->ptep, pte);
> +
> + if (!stage2_try_set_pte(ctx, pte))
> + return -EAGAIN;
> }
>
> return 0;
> @@ -995,7 +1007,7 @@ static int stage2_attr_walker(const struct kvm_pgtable_visit_ctx *ctx,
> static int stage2_update_leaf_attrs(struct kvm_pgtable *pgt, u64 addr,
> u64 size, kvm_pte_t attr_set,
> kvm_pte_t attr_clr, kvm_pte_t *orig_pte,
> - u32 *level)
> + u32 *level, enum kvm_pgtable_walk_flags flags)
> {
> int ret;
> kvm_pte_t attr_mask = KVM_PTE_LEAF_ATTR_LO | KVM_PTE_LEAF_ATTR_HI;
> @@ -1006,7 +1018,7 @@ static int stage2_update_leaf_attrs(struct kvm_pgtable *pgt, u64 addr,
> struct kvm_pgtable_walker walker = {
> .cb = stage2_attr_walker,
> .arg = &data,
> - .flags = KVM_PGTABLE_WALK_LEAF,
> + .flags = flags | KVM_PGTABLE_WALK_LEAF,
> };
>
> ret = kvm_pgtable_walk(pgt, addr, size, &walker);
> @@ -1025,14 +1037,14 @@ int kvm_pgtable_stage2_wrprotect(struct kvm_pgtable *pgt, u64 addr, u64 size)
> {
> return stage2_update_leaf_attrs(pgt, addr, size, 0,
> KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W,
> - NULL, NULL);
> + NULL, NULL, 0);
> }
>
> 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);
> + &pte, NULL, 0);
> dsb(ishst);
> return pte;
> }
> @@ -1041,7 +1053,7 @@ kvm_pte_t kvm_pgtable_stage2_mkold(struct kvm_pgtable *pgt, u64 addr)
> {
> kvm_pte_t pte = 0;
> stage2_update_leaf_attrs(pgt, addr, 1, 0, KVM_PTE_LEAF_ATTR_LO_S2_AF,
> - &pte, NULL);
> + &pte, NULL, 0);
> /*
> * "But where's the TLBI?!", you scream.
> * "Over in the core code", I sigh.
> @@ -1054,7 +1066,7 @@ kvm_pte_t kvm_pgtable_stage2_mkold(struct kvm_pgtable *pgt, u64 addr)
> bool kvm_pgtable_stage2_is_young(struct kvm_pgtable *pgt, u64 addr)
> {
> kvm_pte_t pte = 0;
> - stage2_update_leaf_attrs(pgt, addr, 1, 0, 0, &pte, NULL);
> + stage2_update_leaf_attrs(pgt, addr, 1, 0, 0, &pte, NULL, 0);
Would be nice to have an enum for KVM_PGTABLE_WALK_EXCLUSIVE so this
doesn't just have to pass 0.
> return pte & KVM_PTE_LEAF_ATTR_LO_S2_AF;
> }
>
> @@ -1077,7 +1089,8 @@ int kvm_pgtable_stage2_relax_perms(struct kvm_pgtable *pgt, u64 addr,
> if (prot & KVM_PGTABLE_PROT_X)
> clr |= KVM_PTE_LEAF_ATTR_HI_S2_XN;
>
> - ret = stage2_update_leaf_attrs(pgt, addr, 1, set, clr, NULL, &level);
> + ret = stage2_update_leaf_attrs(pgt, addr, 1, set, clr, NULL, &level,
> + KVM_PGTABLE_WALK_SHARED);
> if (!ret)
> kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, pgt->mmu, addr, level);
> return ret;
> --
> 2.38.1.431.g37b22c650d-goog
>
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