linux-kernel - Re: [PATCH v1] iommu/arm-smmu-v3: Allow default substream bypass with a pasid support

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Message-ID: <826cbb86-ac7d-c40b-f7e3-51681cda50b8@arm.com>
Date:   Tue, 27 Jun 2023 10:00:18 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Nicolin Chen <nicolinc@...dia.com>, will@...nel.org
Cc:     jgg@...dia.com, joro@...tes.org,
        shameerali.kolothum.thodi@...wei.com, yangyicong@...ilicon.com,
        jean-philippe@...aro.org, linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] iommu/arm-smmu-v3: Allow default substream bypass with
 a pasid support

On 2023-06-27 04:33, Nicolin Chen wrote:
> When an iommu_domain is set to IOMMU_DOMAIN_IDENTITY, the driver would
> skip the allocation of a CD table and set the CONFIG field of the STE
> to STRTAB_STE_0_CFG_BYPASS. This works well for devices that only have
> one substream, i.e. PASID disabled.
> 
> However, there could be a use case, for a pasid capable device, that
> allows bypassing the translation at the default substream while still
> enabling the pasid feature, which means the driver should not skip the
> allocation of a CD table nor simply bypass the CONFIG field. Instead,
> the S1DSS field should be set to STRTAB_STE_1_S1DSS_BYPASS and the
> SHCFG field should be set to STRTAB_STE_1_SHCFG_INCOMING.
> 
> Add s1dss and shcfg in struct arm_smmu_s1_cfg, to allow configurations
> in the finalise() to support that use case. Then, set them accordingly
> depending on the iommu_domain->type and the master->ssid_bits.
> 
> Also, add STRTAB_STE_1_SHCFG_NONSHAREABLE of the default configuration
> to distinguish from STRTAB_STE_1_SHCFG_INCOMING of the bypass one.

Why? The "default configuration" is that the S1 shareability attribute 
is determined by the S1 translation itself, so the incoming value is 
irrelevant.

> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> ---
>   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 18 +++++++++++++++---
>   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h |  3 +++
>   2 files changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> index 9b0dc3505601..8dc7934a0175 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -1350,11 +1350,12 @@ static void arm_smmu_write_strtab_ent(struct arm_smmu_master *master, u32 sid,
>   
>   		BUG_ON(ste_live);
>   		dst[1] = cpu_to_le64(
> -			 FIELD_PREP(STRTAB_STE_1_S1DSS, STRTAB_STE_1_S1DSS_SSID0) |
> +			 FIELD_PREP(STRTAB_STE_1_S1DSS, s1_cfg->s1dss) |
>   			 FIELD_PREP(STRTAB_STE_1_S1CIR, STRTAB_STE_1_S1C_CACHE_WBRA) |
>   			 FIELD_PREP(STRTAB_STE_1_S1COR, STRTAB_STE_1_S1C_CACHE_WBRA) |
>   			 FIELD_PREP(STRTAB_STE_1_S1CSH, ARM_SMMU_SH_ISH) |
> -			 FIELD_PREP(STRTAB_STE_1_STRW, strw));
> +			 FIELD_PREP(STRTAB_STE_1_STRW, strw) |
> +			 FIELD_PREP(STRTAB_STE_1_SHCFG, s1_cfg->shcfg));
>   
>   		if (smmu->features & ARM_SMMU_FEAT_STALLS &&
>   		    !master->stall_enabled)
> @@ -2119,6 +2120,13 @@ static int arm_smmu_domain_finalise_s1(struct arm_smmu_domain *smmu_domain,
>   		goto out_unlock;
>   
>   	cfg->s1cdmax = master->ssid_bits;
> +	if (smmu_domain->domain.type == IOMMU_DOMAIN_IDENTITY) {
> +		cfg->s1dss = STRTAB_STE_1_S1DSS_BYPASS;
> +		cfg->shcfg = STRTAB_STE_1_SHCFG_INCOMING;
> +	} else {
> +		cfg->s1dss = STRTAB_STE_1_S1DSS_SSID0;
> +		cfg->shcfg = STRTAB_STE_1_SHCFG_NONSHAREABLE;
> +	}
>   
>   	smmu_domain->stall_enabled = master->stall_enabled;
>   
> @@ -2198,7 +2206,11 @@ static int arm_smmu_domain_finalise(struct iommu_domain *domain,
>   	struct arm_smmu_domain *smmu_domain = to_smmu_domain(domain);
>   	struct arm_smmu_device *smmu = smmu_domain->smmu;
>   
> -	if (domain->type == IOMMU_DOMAIN_IDENTITY) {
> +	/*
> +	 * A master with a pasid capability might need a CD table, so only set
> +	 * ARM_SMMU_DOMAIN_BYPASS if IOMMU_DOMAIN_IDENTITY and non-pasid master
> +	 */
> +	if (domain->type == IOMMU_DOMAIN_IDENTITY && !master->ssid_bits) {
>   		smmu_domain->stage = ARM_SMMU_DOMAIN_BYPASS;
>   		return 0;
>   	}

This means we'll now go on to allocate a pagetable for an identity 
domain, which doesn't seem ideal :/

Thanks,
Robin.

> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
> index dcab85698a4e..8052d02770d0 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
> @@ -244,6 +244,7 @@
>   #define STRTAB_STE_1_STRW_EL2		2UL
>   
>   #define STRTAB_STE_1_SHCFG		GENMASK_ULL(45, 44)
> +#define STRTAB_STE_1_SHCFG_NONSHAREABLE	0UL
>   #define STRTAB_STE_1_SHCFG_INCOMING	1UL
>   
>   #define STRTAB_STE_2_S2VMID		GENMASK_ULL(15, 0)
> @@ -601,6 +602,8 @@ struct arm_smmu_s1_cfg {
>   	struct arm_smmu_ctx_desc_cfg	cdcfg;
>   	struct arm_smmu_ctx_desc	cd;
>   	u8				s1fmt;
> +	u8				s1dss;
> +	u8				shcfg;
>   	u8				s1cdmax;
>   };
>