Re: [PATCH RESEND v4 2/3] KVM: arm64: Introduce KVM_PGTABLE_WALK_SKIP_LEVEL* walk flags

[Oliver Upton <oupton@kernel.org>]

Hi Leo,

On Fri, Nov 06, 2026 at 04:12:37PM +0000, Leonardo Bras wrote:
> Add new walking flags KVM_PGTABLE_WALK_SKIP_LEVEL{0-3}.
> When used, they make the kvm_pgtable_walk() skip the given level, as well
> as levels higher than that.
> 
> Being able to skip levels substancially decreases the runtime of a
> pagetable walk when the callback function does nothing on that level.
> 
> Signed-off-by: Leonardo Bras <leo.bras@arm.com>

I'm still not crazy about using a flag per level... This is what I have
stuffed on top locally. I've got a similar oneline fix for patch #3 to
initialize the field with an offset.

Any opinions?

Thanks,
Oliver

commit cad2070228c4305b5a83ce0a4cfa4aaa3dc0eb44
Author: Oliver Upton <oupton@kernel.org>
Date:   Fri Nov 6 10:03:14 2026 -0800

    squash! KVM: arm64: Introduce KVM_PGTABLE_WALK_SKIP_LEVEL* walk flags
    
    KVM: arm64: Add support for early termination of table walk recursion
    
    Add a new field to the walk flags that represents an offset from the
    last level of the page table hierarchy at which recursion should
    terminate.

    Signed-off-by: Leonardo Bras <leo.bras@arm.com>
    [oliver: pass a numeric value instead of a flag per level]
    Signed-off-by: Oliver Upton <oupton@kernel.org>

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index 1d7a9125e522..18dcce3c506e 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -326,10 +326,8 @@ typedef bool (*kvm_pgtable_force_pte_cb_t)(u64 addr, u64 end,
  * @KVM_PGTABLE_WALK_SKIP_CMO:		Visit and update table entries
  *					without Cache maintenance
  *					operations required.
- * @KVM_PGTABLE_WALK_SKIP_LEVEL0:	Skip visiting level 0+ entries
- * @KVM_PGTABLE_WALK_SKIP_LEVEL1:	Skip visiting level 1+ entries
- * @KVM_PGTABLE_WALK_SKIP_LEVEL2:	Skip visiting level 2+ entries
- * @KVM_PGTABLE_WALK_SKIP_LEVEL3:	Skip visiting level 3 entries
+ * @KVM_PGTABLE_WALK_SKIP_OFFSET:	Offset from KVM_PGTABLE_LAST_LEVEL at
+ * 					which the walk should stop descending
  */
 enum kvm_pgtable_walk_flags {
 	KVM_PGTABLE_WALK_LEAF			= BIT(0),
@@ -339,18 +337,9 @@ enum kvm_pgtable_walk_flags {
 	KVM_PGTABLE_WALK_IGNORE_EAGAIN		= BIT(4),
 	KVM_PGTABLE_WALK_SKIP_BBM_TLBI		= BIT(5),
 	KVM_PGTABLE_WALK_SKIP_CMO		= BIT(6),
-	/* SKIP_LEVEL* flags should not be split nor reordered */
-	KVM_PGTABLE_WALK_SKIP_LEVEL0		= BIT(7),
-	KVM_PGTABLE_WALK_SKIP_LEVEL1		= BIT(8),
-	KVM_PGTABLE_WALK_SKIP_LEVEL2		= BIT(9),
-	KVM_PGTABLE_WALK_SKIP_LEVEL3		= BIT(10),
+	KVM_PGTABLE_WALK_SKIP_OFFSET		= GENMASK(9, 7),
 };
 
-#define KVM_PGTABLE_WALK_SKIP_LEVELS 	(KVM_PGTABLE_WALK_SKIP_LEVEL0 | \
-					 KVM_PGTABLE_WALK_SKIP_LEVEL1 | \
-					 KVM_PGTABLE_WALK_SKIP_LEVEL2 | \
-					 KVM_PGTABLE_WALK_SKIP_LEVEL3 )
-
 struct kvm_pgtable_visit_ctx {
 	kvm_pte_t				*ptep;
 	kvm_pte_t				old;
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 6aaf2696c99a..3008817da6c5 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -147,9 +147,12 @@ static bool kvm_pgtable_walk_continue(const struct kvm_pgtable_walker *walker,
 
 static bool kvm_pgtable_skip_level(s8 level, enum kvm_pgtable_walk_flags flags)
 {
-	u32 skip = FIELD_GET(KVM_PGTABLE_WALK_SKIP_LEVELS, flags);
+	u8 offset = FIELD_GET(KVM_PGTABLE_WALK_SKIP_OFFSET, flags);
 
-	return unlikely(skip && level >= ffs(skip) - 1);
+	BUILD_BUG_ON(!FIELD_FIT(KVM_PGTABLE_WALK_SKIP_OFFSET,
+				KVM_PGTABLE_LAST_LEVEL - KVM_PGTABLE_FIRST_LEVEL));
+
+	return unlikely(level > KVM_PGTABLE_LAST_LEVEL - offset);
 }
 
 static int __kvm_pgtable_walk(struct kvm_pgtable_walk_data *data,

> ---
>  arch/arm64/include/asm/kvm_pgtable.h | 14 ++++++++++++++
>  arch/arm64/kvm/hyp/pgtable.c         | 16 +++++++++++++---
>  2 files changed, 27 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index 41a8687938eb..5f37ceef8d63 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -311,31 +311,45 @@ typedef bool (*kvm_pgtable_force_pte_cb_t)(u64 addr, u64 end,
>   * @KVM_PGTABLE_WALK_SHARED:		Indicates the page-tables may be shared
>   *					with other software walkers.
>   * @KVM_PGTABLE_WALK_IGNORE_EAGAIN:	Don't terminate the walk early if
>   *					the walker returns -EAGAIN.
>   * @KVM_PGTABLE_WALK_SKIP_BBM_TLBI:	Visit and update table entries
>   *					without Break-before-make's
>   *					TLB invalidation.
>   * @KVM_PGTABLE_WALK_SKIP_CMO:		Visit and update table entries
>   *					without Cache maintenance
>   *					operations required.
> + * @KVM_PGTABLE_WALK_SKIP_LEVEL0:	Skip visiting level 0+ entries
> + * @KVM_PGTABLE_WALK_SKIP_LEVEL1:	Skip visiting level 1+ entries
> + * @KVM_PGTABLE_WALK_SKIP_LEVEL2:	Skip visiting level 2+ entries
> + * @KVM_PGTABLE_WALK_SKIP_LEVEL3:	Skip visiting level 3 entries
>   */
>  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_IGNORE_EAGAIN		= BIT(4),
>  	KVM_PGTABLE_WALK_SKIP_BBM_TLBI		= BIT(5),
>  	KVM_PGTABLE_WALK_SKIP_CMO		= BIT(6),
> +	/* SKIP_LEVEL* flags should not be split nor reordered */
> +	KVM_PGTABLE_WALK_SKIP_LEVEL0		= BIT(7),
> +	KVM_PGTABLE_WALK_SKIP_LEVEL1		= BIT(8),
> +	KVM_PGTABLE_WALK_SKIP_LEVEL2		= BIT(9),
> +	KVM_PGTABLE_WALK_SKIP_LEVEL3		= BIT(10),
>  };
>  
> +#define KVM_PGTABLE_WALK_SKIP_LEVELS 	(KVM_PGTABLE_WALK_SKIP_LEVEL0 | \
> +					 KVM_PGTABLE_WALK_SKIP_LEVEL1 | \
> +					 KVM_PGTABLE_WALK_SKIP_LEVEL2 | \
> +					 KVM_PGTABLE_WALK_SKIP_LEVEL3 )
> +
>  struct kvm_pgtable_visit_ctx {
>  	kvm_pte_t				*ptep;
>  	kvm_pte_t				old;
>  	void					*arg;
>  	struct kvm_pgtable_mm_ops		*mm_ops;
>  	u64					start;
>  	u64					addr;
>  	u64					end;
>  	s8					level;
>  	enum kvm_pgtable_walk_flags		flags;
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 7a51f78b6446..80744478c810 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -137,20 +137,27 @@ static bool kvm_pgtable_walk_continue(const struct kvm_pgtable_walker *walker,
>  	 * 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_IGNORE_EAGAIN;
>  
>  	return !r;
>  }
>  
> +static bool kvm_pgtable_skip_level(s8 level, enum kvm_pgtable_walk_flags flags)
> +{
> +	u32 skip = FIELD_GET(KVM_PGTABLE_WALK_SKIP_LEVELS, flags);
> +
> +	return unlikely(skip && level >= ffs(skip) - 1);
> +}
> +
>  static int __kvm_pgtable_walk(struct kvm_pgtable_walk_data *data,
>  			      struct kvm_pgtable_mm_ops *mm_ops, kvm_pteref_t pgtable, s8 level);
>  
>  static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
>  				      struct kvm_pgtable_mm_ops *mm_ops,
>  				      kvm_pteref_t pteref, s8 level)
>  {
>  	enum kvm_pgtable_walk_flags flags = data->walker->flags;
>  	kvm_pte_t *ptep = kvm_dereference_pteref(data->walker, pteref);
>  	struct kvm_pgtable_visit_ctx ctx = {
> @@ -185,35 +192,35 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
>  	 * into a newly installed or replaced table.
>  	 */
>  	if (reload) {
>  		ctx.old = READ_ONCE(*ptep);
>  		table = kvm_pte_table(ctx.old, level);
>  	}
>  
>  	if (!kvm_pgtable_walk_continue(data->walker, ret))
>  		return ret;
>  
> -	if (!table) {
> +	if (!table || kvm_pgtable_skip_level(level + 1, ctx.flags)) {
>  		data->addr = ALIGN_DOWN(data->addr, kvm_granule_size(level));
>  		data->addr += kvm_granule_size(level);
>  		goto out;
>  	}
>  
>  	childp = (kvm_pteref_t)kvm_pte_follow(ctx.old, mm_ops);
>  	ret = __kvm_pgtable_walk(data, mm_ops, childp, level + 1);
>  	if (!kvm_pgtable_walk_continue(data->walker, ret))
>  		return ret;
>  
> -	if (ctx.flags & KVM_PGTABLE_WALK_TABLE_POST)
> +out:
> +	if (table && 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;
>  }
>  
>  static int __kvm_pgtable_walk(struct kvm_pgtable_walk_data *data,
>  			      struct kvm_pgtable_mm_ops *mm_ops, kvm_pteref_t pgtable, s8 level)
>  {
>  	u32 idx;
> @@ -242,20 +249,23 @@ static int _kvm_pgtable_walk(struct kvm_pgtable *pgt, struct kvm_pgtable_walk_da
>  	u32 idx;
>  	int ret = 0;
>  	u64 limit = BIT(pgt->ia_bits);
>  
>  	if (data->addr > limit || data->end > limit)
>  		return -ERANGE;
>  
>  	if (!pgt->pgd)
>  		return -EINVAL;
>  
> +	if (kvm_pgtable_skip_level(pgt->start_level, data->walker->flags))
> +		return 0;
> +
>  	for (idx = kvm_pgd_page_idx(pgt, data->addr); data->addr < data->end; ++idx) {
>  		kvm_pteref_t pteref = &pgt->pgd[idx * PTRS_PER_PTE];
>  
>  		ret = __kvm_pgtable_walk(data, pgt->mm_ops, pteref, pgt->start_level);
>  		if (ret)
>  			break;
>  	}
>  
>  	return ret;
>  }
> -- 
> 2.55.0
>