Re: [PATCH v2 14/16] KVM: arm64: Add S1 IPA to page table level walker

[Oliver Upton <oliver.upton@linux.dev>]

On Mon, Sep 15, 2025 at 12:44:49PM +0100, Marc Zyngier wrote:
> Use the filtering hook infrastructure to implement a new walker
> that, for a given VA and an IPA, returns the level of the first
> occurence of this IPA in the walk from that VA.
> 
> This will be used to improve our SEA syndrome reporting.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/include/asm/kvm_nested.h |  2 +
>  arch/arm64/kvm/at.c                 | 65 +++++++++++++++++++++++++++++
>  2 files changed, 67 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/kvm_nested.h b/arch/arm64/include/asm/kvm_nested.h
> index cce0e4cb54484..2be6c3de74e3d 100644
> --- a/arch/arm64/include/asm/kvm_nested.h
> +++ b/arch/arm64/include/asm/kvm_nested.h
> @@ -353,6 +353,8 @@ struct s1_walk_result {
>  
>  int __kvm_translate_va(struct kvm_vcpu *vcpu, struct s1_walk_info *wi,
>  		       struct s1_walk_result *wr, u64 va);
> +int __kvm_find_s1_desc_level(struct kvm_vcpu *vcpu, u64 va, u64 ipa,
> +			     int *level);
>  
>  /* VNCR management */
>  int kvm_vcpu_allocate_vncr_tlb(struct kvm_vcpu *vcpu);
> diff --git a/arch/arm64/kvm/at.c b/arch/arm64/kvm/at.c
> index c099bab30cb0d..117ce1ff5e767 100644
> --- a/arch/arm64/kvm/at.c
> +++ b/arch/arm64/kvm/at.c
> @@ -1578,3 +1578,68 @@ int __kvm_translate_va(struct kvm_vcpu *vcpu, struct s1_walk_info *wi,
>  
>  	return 0;
>  }
> +
> +struct desc_match {
> +	u64	ipa;
> +	int	level;
> +};
> +
> +static int match_s1_desc(struct s1_walk_context *ctxt, void *priv)
> +{
> +	struct desc_match *dm = priv;
> +	u64 ipa = dm->ipa;
> +
> +	/* Use S1 granule alignment */
> +	ipa &= GENMASK(52, ctxt->wi->pgshift);
> +

Bit 51 again

> +	/* Not the IPA we're looking for? Continue. */
> +	if (ipa != ctxt->table_ipa)
> +		return 0;
> +
> +	/* Note the level and interrupt the walk */
> +	dm->level = ctxt->level;
> +	return -EINTR;
> +}
> +
> +int __kvm_find_s1_desc_level(struct kvm_vcpu *vcpu, u64 va, u64 ipa, int *level)
> +{
> +	struct desc_match dm = {
> +		.ipa	= ipa,
> +	};
> +	struct s1_walk_info wi = {
> +		.filter	= &(struct s1_walk_filter){
> +			.fn	= match_s1_desc,
> +			.priv	= &dm,
> +		},
> +		.regime	= TR_EL10,
> +		.as_el0	= false,
> +		.pan	= false,
> +	};
> +	struct s1_walk_result wr = {};
> +	int ret;
> +
> +	ret = setup_s1_walk(vcpu, &wi, &wr, va);
> +	if (ret)
> +		return ret;
> +
> +	/* We really expect the S1 MMU to be on here... */
> +	if (WARN_ON_ONCE(wr.level == S1_MMU_DISABLED)) {
> +		*level = 0;
> +		return 0;
> +	}
> +
> +	/* Walk the guest's PT, looking for a match along the way */
> +	ret = walk_s1(vcpu, &wi, &wr, va);
> +	switch (ret) {
> +	case -EINTR:
> +		/* We interrupted the walk on a match, return the level */
> +		*level = dm.level;
> +		return 0;
> +	case 0:
> +		/* The walk completed, we failed to find the entry */
> +		return -ENOENT;
> +	default:
> +		/* Any other error... */
> +		return ret;
> +	}
> +}
> -- 
> 2.39.2
>