linux-kernel - Re: [PATCH 4/8] iommu/arm-smmu-v3: Add support for Substream IDs

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Date:   Wed, 26 Jun 2019 19:00:26 +0100
From:   Will Deacon <will@...nel.org>
To:     Jean-Philippe Brucker <jean-philippe.brucker@....com>
Cc:     will.deacon@....com, joro@...tes.org, robh+dt@...nel.org,
        mark.rutland@....com, robin.murphy@....com,
        jacob.jun.pan@...ux.intel.com, iommu@...ts.linux-foundation.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, eric.auger@...hat.com
Subject: Re: [PATCH 4/8] iommu/arm-smmu-v3: Add support for Substream IDs

On Mon, Jun 10, 2019 at 07:47:10PM +0100, Jean-Philippe Brucker wrote:
> At the moment, the SMMUv3 driver implements only one stage-1 or stage-2
> page directory per device. However SMMUv3 allows more than one address
> space for some devices, by providing multiple stage-1 page directories. In
> addition to the Stream ID (SID), that identifies a device, we can now have
> Substream IDs (SSID) identifying an address space. In PCIe, SID is called
> Requester ID (RID) and SSID is called Process Address-Space ID (PASID).
> 
> Prepare the driver for SSID support, by adding context descriptor tables
> in STEs (previously a single static context descriptor). A complete
> stage-1 walk is now performed like this by the SMMU:
> 
>       Stream tables          Ctx. tables          Page tables
>         +--------+   ,------->+-------+   ,------->+-------+
>         :        :   |        :       :   |        :       :
>         +--------+   |        +-------+   |        +-------+
>    SID->|  STE   |---'  SSID->|  CD   |---'  IOVA->|  PTE  |--> IPA
>         +--------+            +-------+            +-------+
>         :        :            :       :            :       :
>         +--------+            +-------+            +-------+
> 
> Implement a single level of context descriptor table for now, but as with
> stream and page tables, an SSID can be split to index multiple levels of
> tables.
> 
> In all stream table entries, we set S1DSS=SSID0 mode, making translations
> without an SSID use context descriptor 0. Although it would be possible by
> setting S1DSS=BYPASS, we don't currently support SSID when user selects
> iommu.passthrough.

I don't understand your comment here: iommu.passthrough works just as it did
before, right, since we set bypass in the STE config field so S1DSS is not
relevant? I also notice that SSID0 causes transactions with SSID==0 to
abort. Is a PASID of 0 reserved, so this doesn't matter?

> @@ -1062,33 +1143,90 @@ static u64 arm_smmu_cpu_tcr_to_cd(u64 tcr)
>  	return val;
>  }
>  
> -static void arm_smmu_write_ctx_desc(struct arm_smmu_device *smmu,
> -				    struct arm_smmu_s1_cfg *cfg)
> +static int arm_smmu_write_ctx_desc(struct arm_smmu_domain *smmu_domain,
> +				   int ssid, struct arm_smmu_ctx_desc *cd)
>  {
>  	u64 val;
> +	bool cd_live;
> +	struct arm_smmu_device *smmu = smmu_domain->smmu;
> +	__le64 *cdptr = arm_smmu_get_cd_ptr(&smmu_domain->s1_cfg, ssid);
>  
>  	/*
> -	 * We don't need to issue any invalidation here, as we'll invalidate
> -	 * the STE when installing the new entry anyway.
> +	 * This function handles the following cases:
> +	 *
> +	 * (1) Install primary CD, for normal DMA traffic (SSID = 0).
> +	 * (2) Install a secondary CD, for SID+SSID traffic.
> +	 * (3) Update ASID of a CD. Atomically write the first 64 bits of the
> +	 *     CD, then invalidate the old entry and mappings.
> +	 * (4) Remove a secondary CD.
>  	 */
> -	val = arm_smmu_cpu_tcr_to_cd(cfg->cd.tcr) |
> +
> +	if (!cdptr)
> +		return -ENOMEM;
> +
> +	val = le64_to_cpu(cdptr[0]);
> +	cd_live = !!(val & CTXDESC_CD_0_V);
> +
> +	if (!cd) { /* (4) */
> +		cdptr[0] = 0;

Should we be using WRITE_ONCE here? (although I notice we don't seem to
bother for STEs either...)

Will