linux-kernel - Re: [PATCH 1/2] iommu/arm-smmu-v3: Do not bother impl_ops if IOMMU_VIOMMU_TYPE_ARM

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Message-ID: <aH4-wuIjtjyo3aOo@willie-the-truck>
Date: Mon, 21 Jul 2025 14:21:06 +0100
From: Will Deacon <will@...nel.org>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: jgg@...dia.com, joro@...tes.org, robin.murphy@....com,
	linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org, linux-tegra@...r.kernel.org
Subject: Re: [PATCH 1/2] iommu/arm-smmu-v3: Do not bother impl_ops if
 IOMMU_VIOMMU_TYPE_ARM_SMMUV3

On Fri, Jul 18, 2025 at 04:48:21PM -0700, Nicolin Chen wrote:
> When viommu type is IOMMU_VIOMMU_TYPE_ARM_SMMUV3, always return or init the
> standard struct arm_vsmmu, instead of going through impl_ops that must have
> its own viommu type than the standard IOMMU_VIOMMU_TYPE_ARM_SMMUV3.
> 
> Given that arm_vsmmu_init() is called after arm_smmu_get_viommu_size(), any
> unsupported viommu->type must be a corruption. And missing a pairing impl's
> vsmmu_init should be a driver bug too. Warn these two cases.
> 
> Suggested-by: Will Deacon <will@...nel.org>
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> ---
>  .../arm/arm-smmu-v3/arm-smmu-v3-iommufd.c     | 30 ++++++++++++-------
>  1 file changed, 19 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
> index d9bea8f1f636..0b2acb80f41b 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
> @@ -420,14 +420,13 @@ size_t arm_smmu_get_viommu_size(struct device *dev,
>  	    !(smmu->features & ARM_SMMU_FEAT_S2FWB))
>  		return 0;
>  
> -	if (smmu->impl_ops && smmu->impl_ops->vsmmu_size &&
> -	    viommu_type == smmu->impl_ops->vsmmu_type)
> -		return smmu->impl_ops->vsmmu_size;
> +	if (viommu_type == IOMMU_VIOMMU_TYPE_ARM_SMMUV3)
> +		return VIOMMU_STRUCT_SIZE(struct arm_vsmmu, core);
>  
> -	if (viommu_type != IOMMU_VIOMMU_TYPE_ARM_SMMUV3)
> +	if (!smmu->impl_ops || !smmu->impl_ops->vsmmu_size ||
> +	    viommu_type != smmu->impl_ops->vsmmu_type)
>  		return 0;
> -
> -	return VIOMMU_STRUCT_SIZE(struct arm_vsmmu, core);
> +	return smmu->impl_ops->vsmmu_size;
>  }
>  
>  int arm_vsmmu_init(struct iommufd_viommu *viommu,
> @@ -447,12 +446,21 @@ int arm_vsmmu_init(struct iommufd_viommu *viommu,
>  	/* FIXME Move VMID allocation from the S2 domain allocation to here */
>  	vsmmu->vmid = s2_parent->s2_cfg.vmid;
>  
> -	if (smmu->impl_ops && smmu->impl_ops->vsmmu_init &&
> -	    viommu->type == smmu->impl_ops->vsmmu_type)
> -		return smmu->impl_ops->vsmmu_init(vsmmu, user_data);
> +	if (viommu->type == IOMMU_VIOMMU_TYPE_ARM_SMMUV3) {
> +		viommu->ops = &arm_vsmmu_ops;
> +		return 0;
> +	}
>  
> -	viommu->ops = &arm_vsmmu_ops;
> -	return 0;
> +	/*
> +	 * If a non standard type was supported in arm_smmu_get_viommu_size() by
> +	 * an implementation, a pairing vsmmu_init op must exist.
> +	 */
> +	if (WARN_ON_ONCE(!smmu->impl_ops || !smmu->impl_ops->vsmmu_init))
> +		return -EOPNOTSUPP;

It might be cleaner to check the impl_ops in arm_smmu_impl_probe() where
we're in a slightly better situation for failing gracefully and
deterministically.

Otherwise, looks good. Thanks!

Will