Re: [PATCH v2 01/22] KVM: arm64: Prefault host stage-2 entries on block split

[Mostafa Saleh <smostafa@google.com>]

On Fri, Sep 11, 2026 at 02:50:32PM +0100, 'Vincent Donnefort' via kernel-team wrote:
> From: Keir Fraser <keirf@google.com>
> 
> For the host stage-2 that is idmap, we can avoid lazy-mapping on block
> splitting by propagating existing valid mappings to neighbouring entries
> of the new table.
> 
> Signed-off-by: Keir Fraser <keirf@google.com>
> Co-developed-by: Quentin Perret <qperret@google.com>
> Signed-off-by: Quentin Perret <qperret@google.com>
> Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> ---
>  arch/arm64/kvm/hyp/pgtable.c | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index b74dd5ce1efd..34f78a58cded 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -1030,10 +1030,39 @@ static int stage2_map_walk_table_pre(const struct kvm_pgtable_visit_ctx *ctx,
>  	return 0;
>  }
>  
> +static void stage2_map_prefault_idmap(const struct kvm_pgtable_visit_ctx *ctx, kvm_pte_t *ptep)
> +{
> +	kvm_pte_t block_pte = ctx->old;
> +	u64 pa;
> +	int i;
> +
> +	if (!kvm_pte_valid(block_pte))
> +		return;
> +
> +	pa = ALIGN_DOWN(ctx->addr, kvm_granule_size(ctx->level));
> +	for (i = 0; i < PTRS_PER_PTE; ++i, ++ptep, pa += kvm_granule_size(ctx->level + 1)) {
> +		kvm_pte_t pte = kvm_init_valid_leaf_pte(pa, block_pte, ctx->level + 1);
> +
> +		/*
> +		 * Skip ptes in the range being modified by the caller if we're
> +		 * installing last level entries. Otherwise, we need to
> +		 * temporarily put in a valid mapping to make sure the
> +		 * prefaulting logic is triggered on the next
> +		 * stage2_map_walk_leaf(). This adds an unnecessary TLBI as
> +		 * we'll presumably re-break the freshly installed block, but
> +		 * that should happen very infrequently.
> +		 */
> +		if ((ctx->level < (KVM_PGTABLE_LAST_LEVEL - 1)) ||
> +		    (pa < ctx->addr) || (pa >= ctx->end))
> +			*ptep = pte;

Do we need now to call get_page() on the table as we are installing a
new PTE?

Also, I think we need a WRITE_ONCE() for the PTE write.

Thanks,
Mostafa

> +	}
> +}
> +
>  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;
> +	struct kvm_pgtable *pgt = data->mmu->pgt;
>  	kvm_pte_t *childp, new;
>  	int ret;
>  
> @@ -1051,6 +1080,9 @@ static int stage2_map_walk_leaf(const struct kvm_pgtable_visit_ctx *ctx,
>  	if (!childp)
>  		return -ENOMEM;
>  
> +	if (pgt->flags & KVM_PGTABLE_S2_IDMAP)
> +		stage2_map_prefault_idmap(ctx, childp);
> +
>  	if (!stage2_try_break_pte(ctx, data->mmu)) {
>  		mm_ops->put_page(childp);
>  		return -EAGAIN;
> -- 
> 2.55.0.1007.g17ff1f9808-goog
> 
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