Re: [PATCH v3 07/13] iommu/arm-smmu-v3: Add support for Substream IDs - Jean-Philippe Brucker

From: Jean-Philippe Brucker <jean-philippe@linaro.org>
To: Auger Eric <eric.auger@redhat.com>
Cc: linux-pci@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-acpi@vger.kernel.org, devicetree@vger.kernel.org,
	iommu@lists.linux-foundation.org, joro@8bytes.org,
	robh+dt@kernel.org, mark.rutland@arm.com,
	lorenzo.pieralisi@arm.com, guohanjun@huawei.com,
	sudeep.holla@arm.com, rjw@rjwysocki.net, lenb@kernel.org,
	will@kernel.org, robin.murphy@arm.com, bhelgaas@google.com,
	jonathan.cameron@huawei.com, zhangfei.gao@linaro.org
Subject: Re: [PATCH v3 07/13] iommu/arm-smmu-v3: Add support for Substream IDs
Date: Wed, 18 Dec 2019 17:07:59 +0100	[thread overview]
Message-ID: <20191218160759.GF2371701@myrica> (raw)
In-Reply-To: <aeed148a-3c55-6e97-8d97-efa4f8c6d9f4@redhat.com>

On Tue, Dec 17, 2019 at 05:43:59PM +0100, Auger Eric wrote:
> > -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;
> > -	__le64 *cdptr = cfg->table.ptr;
> > -
> >  	/*
> > -	 * 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.
> I see arm_smmu_write_ctx_desc getting called with non null cd in
> arm_smmu_attach_dev but I do not see it removed in the detach path?

No we don't have to remove the primary CD, since detach clears the STE and
frees the CD tables.

> >  	 */
> > -	val = arm_smmu_cpu_tcr_to_cd(cfg->cd.tcr) |
> > -#ifdef __BIG_ENDIAN
> > -	      CTXDESC_CD_0_ENDI |
> > -#endif
> > -	      CTXDESC_CD_0_R | CTXDESC_CD_0_A | CTXDESC_CD_0_ASET |
> > -	      CTXDESC_CD_0_AA64 | FIELD_PREP(CTXDESC_CD_0_ASID, cfg->cd.asid) |
> > -	      CTXDESC_CD_0_V;
> > +	u64 val;
> > +	bool cd_live;
> > +	struct arm_smmu_device *smmu = smmu_domain->smmu;
> > +	__le64 *cdptr = smmu_domain->s1_cfg.table.ptr + ssid *
> > +			CTXDESC_CD_DWORDS;>
> > -	/* STALL_MODEL==0b10 && CD.S==0 is ILLEGAL */
> > -	if (smmu->features & ARM_SMMU_FEAT_STALL_FORCE)
> > -		val |= CTXDESC_CD_0_S;
> > +	val = le64_to_cpu(cdptr[0]);
> > +	cd_live = !!(val & CTXDESC_CD_0_V);
> >  
> > -	cdptr[0] = cpu_to_le64(val);
> > +	if (!cd) { /* (4) */
> > +		val = 0;
> > +	} else if (cd_live) { /* (3) */
> > +		val &= ~CTXDESC_CD_0_ASID;
> > +		val |= FIELD_PREP(CTXDESC_CD_0_ASID, cd->asid);
> > +		/*
> > +		 * Until CD+TLB invalidation, both ASIDs may be used for tagging
> > +		 * this substream's traffic
> > +		 */
> > +	} else { /* (1) and (2) */
> > +		cdptr[1] = cpu_to_le64(cd->ttbr & CTXDESC_CD_1_TTB0_MASK);
> > +		cdptr[2] = 0;
> > +		cdptr[3] = cpu_to_le64(cd->mair);
> > +
> > +		/*
> > +		 * STE is live, and the SMMU might fetch this CD at any
> > +		 * time. Ensure that it observes the rest of the CD before we
> > +		 * enable it.
> Mostly, on't you want the invalid state to be seen?

Sorry I didn't get your question. Without the sync, the SMMU could read an
invalid address in CD[1], read V=1 in CD[0] and proceed with dereferencing
the bogus pointer before seeing our update to CD[1].

To prevent this we follow the update procedure described in IHI0070Ca
3.21.3.1 (Configuration structure update procedure):

    Because the SMMU can read any reachable structure at any time, and is not
    required to read the double-words of the structure in order, Arm
    recommends that the following procedure is used to initialize structures:
    1. Structure starts invalid, having V == 0.
    2. Fill in all fields, leaving V == 0, then perform a DSB operation to
       ensure written data is observable from the SMMU.
    3. Issue a CMD_CFGI_<STRUCT>, as appropriate.
    4. Issue a CMD_SYNC, and wait for completion.
    5. Set V to 1, then perform a DSB operation to ensure write is
       observable by the SMMU.
    6. Issue CMD_CFGI_<STRUCT>, as appropriate.
    7. Optionally issue a CMD_SYNC, and wait for completion. This must be
       done if a subsequent software operation, such as enabling device
       DMA, depends on the SMMU using the new structure.

> > @@ -1664,6 +1739,7 @@ static void arm_smmu_write_strtab_ent(struct arm_smmu_master *master, u32 sid,
> >  	if (s1_cfg) {
> >  		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_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) |
> > @@ -1674,7 +1750,9 @@ static void arm_smmu_write_strtab_ent(struct arm_smmu_master *master, u32 sid,
> >  			dst[1] |= cpu_to_le64(STRTAB_STE_1_S1STALLD);
> >  
> >  		val |= (s1_cfg->table.ptr_dma & STRTAB_STE_0_S1CTXPTR_MASK) |
> > -			FIELD_PREP(STRTAB_STE_0_CFG, STRTAB_STE_0_CFG_S1_TRANS);
> > +			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);
> Do I understand correctly that each time we update the CD, we also
> update the STE. This is not wrong but theoretically this is not needed,
> right?

Only the primary CD. Once we add SVA (or auxiliary domains) we need to
update secondary CDs while the STE is live. This series is just
preparation for SVA support. I didn't include the full support because it
requires another 25+ patches.

> This also explains why you do not need the CMD_PREFETCH_CONFIG with ssid
> because you invalidate the config per sid?

Hm, I didn't add a CMD_PREFETCH_CONFIG to CDs because I don't know if it's
useful. Maybe as future optimization.

Thanks,
Jean