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

Jean-Philippe Brucker
2019-07-04T09:34:31Z

On 26/06/2019 19:00, Will Deacon wrote:
> 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?

Yes the comment is wrong, or at least unclear. It isn't well defined how
SSID is supposed to work with iommu.passthrough, but I guess keeping the
same behavior as non-PASID DMA is what we want (any PASID-tagged DMA
also bypasses the SMMU.)

In the comment I was referring to another possibility, supporting SVA
and auxiliary domains even when iommu.passthrough is set. That would
require allocating context tables and setting S1DSS=BYPASS. But I don't
think it's a feature anyone needs at the moment.

> I also notice that SSID0 causes transactions with SSID==0 to
> abort. Is a PASID of 0 reserved, so this doesn't matter?

Yes, PASID 0 is reserved, we start allocation at 1

>
>> @@ -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...)

Sure, that's safer

Thanks,
Jean