linux-kernel - Re: [PATCH v5 2/9] iommu/arm-smmu-v3: Replace s1_cfg with cdtab

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Message-ID: <20230809134941.GA4226@willie-the-truck>
Date:   Wed, 9 Aug 2023 14:49:41 +0100
From:   Will Deacon <will@...nel.org>
To:     Michael Shavit <mshavit@...gle.com>
Cc:     iommu@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, robin.murphy@....com,
        nicolinc@...dia.com, jgg@...dia.com, jean-philippe@...aro.org
Subject: Re: [PATCH v5 2/9] iommu/arm-smmu-v3: Replace s1_cfg with cdtab_cfg

On Wed, Aug 09, 2023 at 01:11:58AM +0800, Michael Shavit wrote:
> Remove struct arm_smmu_s1_cfg. This is really just a CD table with a
> bit of extra information. Enhance the existing CD table structure,
> struct arm_smmu_ctx_desc_cfg, with max_cds_bits and replace all usages
> of arm_smmu_s1_cfg with arm_smmu_ctx_desc_cfg.
> 
> Compute the other values that were stored in s1cfg directly from
> existing arm_smmu_ctx_desc_cfg.
> 
> For clarity, use the name "cd_table" for the variables pointing to
> arm_smmu_ctx_desc_cfg in the new code instead of cdcfg. A later patch
> will make this fully consistent.
> 
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> Reviewed-by: Nicolin Chen <nicolinc@...dia.com>
> Signed-off-by: Michael Shavit <mshavit@...gle.com>
> ---

Sorry, but I'm having a hard time seeing some of the benefits of this
particular change. Most of the rest of the series looks good, but see
below:

> @@ -1071,7 +1071,7 @@ int arm_smmu_write_ctx_desc(struct arm_smmu_domain *smmu_domain, int ssid,
>  	bool cd_live;
>  	__le64 *cdptr;
>  
> -	if (WARN_ON(ssid >= (1 << smmu_domain->s1_cfg.s1cdmax)))
> +	if (WARN_ON(ssid >= (1 << smmu_domain->cd_table.max_cds_bits)))
>  		return -E2BIG;

S1CDMAX is architectural terminology -- it's the name given to bits 63:59
of the STE structure. Why is "max_cds_bits" better?

>  	cdptr = arm_smmu_get_cd_ptr(smmu_domain, ssid);
> @@ -1138,19 +1138,16 @@ static int arm_smmu_alloc_cd_tables(struct arm_smmu_domain *smmu_domain)
>  	size_t l1size;
>  	size_t max_contexts;
>  	struct arm_smmu_device *smmu = smmu_domain->smmu;
> -	struct arm_smmu_s1_cfg *cfg = &smmu_domain->s1_cfg;
> -	struct arm_smmu_ctx_desc_cfg *cdcfg = &cfg->cdcfg;
> +	struct arm_smmu_ctx_desc_cfg *cdcfg = &smmu_domain->cd_table;
>  
> -	max_contexts = 1 << cfg->s1cdmax;
> +	max_contexts = 1 << cdcfg->max_cds_bits;
>  
>  	if (!(smmu->features & ARM_SMMU_FEAT_2_LVL_CDTAB) ||
>  	    max_contexts <= CTXDESC_L2_ENTRIES) {
> -		cfg->s1fmt = STRTAB_STE_0_S1FMT_LINEAR;
>  		cdcfg->num_l1_ents = max_contexts;
>  
>  		l1size = max_contexts * (CTXDESC_CD_DWORDS << 3);
>  	} else {
> -		cfg->s1fmt = STRTAB_STE_0_S1FMT_64K_L2;

And here we're dropping the S1FMT setting from the code allocating the
CD tables (i.e. the only code which should be aware of it's configuration)
and now having the low-level STE writing logic here:

> @@ -1360,10 +1357,14 @@ static void arm_smmu_write_strtab_ent(struct arm_smmu_master *master, u32 sid,
>  		    !master->stall_enabled)
>  			dst[1] |= cpu_to_le64(STRTAB_STE_1_S1STALLD);
>  
> -		val |= (s1_cfg->cdcfg.cdtab_dma & STRTAB_STE_0_S1CTXPTR_MASK) |
> -			FIELD_PREP(STRTAB_STE_0_CFG, STRTAB_STE_0_CFG_S1_TRANS) |
> -			FIELD_PREP(STRTAB_STE_0_S1CDMAX, s1_cfg->s1cdmax) |
> -			FIELD_PREP(STRTAB_STE_0_S1FMT, s1_cfg->s1fmt);
> +		val |= (cd_table->cdtab_dma & STRTAB_STE_0_S1CTXPTR_MASK) |
> +		       FIELD_PREP(STRTAB_STE_0_CFG, STRTAB_STE_0_CFG_S1_TRANS) |
> +		       FIELD_PREP(STRTAB_STE_0_S1CDMAX,
> +				  cd_table->max_cds_bits) |
> +		       FIELD_PREP(STRTAB_STE_0_S1FMT,
> +				  cd_table->l1_desc ?
> +					  STRTAB_STE_0_S1FMT_64K_L2 :
> +					  STRTAB_STE_0_S1FMT_LINEAR);

magically know that we're using 64k tables.

Why is this an improvement to the driver?

Will