linux-kernel - RE: [PATCH v3 1/2] iommu/vt-d: Add helper to flush caches for context change

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Date: Tue, 2 Jul 2024 06:39:42 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: "Liu, Yi L" <yi.l.liu@...el.com>, Baolu Lu <baolu.lu@...ux.intel.com>,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>, Robin Murphy
	<robin.murphy@....com>, Jason Gunthorpe <jgg@...pe.ca>
CC: "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v3 1/2] iommu/vt-d: Add helper to flush caches for context
 change

> From: Liu, Yi L <yi.l.liu@...el.com>
> Sent: Tuesday, July 2, 2024 2:25 PM
> 
> On 2024/7/2 12:51, Baolu Lu wrote:
> > On 2024/7/2 10:43, Baolu Lu wrote:
> >> On 7/2/24 9:47 AM, Baolu Lu wrote:
> >>> On 7/2/24 9:11 AM, Tian, Kevin wrote:
> >>>>> From: Lu Baolu<baolu.lu@...ux.intel.com>
> >>>>> Sent: Monday, July 1, 2024 7:23 PM
> >>>>> +
> >>>>> +    /*
> >>>>> +     * For scalable mode:
> >>>>> +     * - Domain-selective PASID-cache invalidation to affected domains
> >>>>> +     * - Domain-selective IOTLB invalidation to affected domains
> >>>>> +     * - Global Device-TLB invalidation to affected functions
> >>>>> +     */
> >>>>> +    if (flush_domains) {
> >>>>> +        /*
> >>>>> +         * If the IOMMU is running in scalable mode and there might
> >>>>> +         * be potential PASID translations, the caller should hold
> >>>>> +         * the lock to ensure that context changes and cache flushes
> >>>>> +         * are atomic.
> >>>>> +         */
> >>>>> +        assert_spin_locked(&iommu->lock);
> >>>>> +        for (i = 0; i < info->pasid_table->max_pasid; i++) {
> >>>>> +            pte = intel_pasid_get_entry(info->dev, i);
> >>>>> +            if (!pte || !pasid_pte_is_present(pte))
> >>>>> +                continue;
> >>>>> +
> >>>>> +            did = pasid_get_domain_id(pte);
> >>>>> +            qi_flush_pasid_cache(iommu, did,
> >>>>> QI_PC_ALL_PASIDS, 0);
> >>>>> +            iommu->flush.flush_iotlb(iommu, did, 0, 0,
> >>>>> DMA_TLB_DSI_FLUSH);
> >>>>> +        }
> >>>>> +    }
> >>>>> +
> >>>>> +    __context_flush_dev_iotlb(info);
> >>>>> +}
> >>>> this only invalidates devtlb w/o PASID. We miss a pasid devtlb
> >>>> invalidation
> >>>> with global bit set.
> >>>
> >>> I am not sure about this. The spec says "Global Device-TLB invalidation
> >>> to affected functions", I am not sure whether this implies any PASID-
> >>> based-Device-TLB invalidation.
> >>
> >> I just revisited the spec, Device-TLB invalidation only covers caches
> >> for requests-without-PASID. If pasid translation is affected while
> >> updating the context entry, we should also take care of the caches for
> >> requests-with-pasid.
> >>
> 
> hmmm. "Table 25. Guidance to Software for Invalidations" only mentions
> global devTLB invalidation. 10.3.8 PASID and Global Invalidate of PCIe 6.2
> spec has below definition.
> 
> For Invalidation Requests that do not have a PASID, the ATC shall:
> • Invalidate ATC entries within the indicate memory range that were
> requested without a PASID value.
> • Invalidate ATC entries at all addresses that were requested with any
> PASID value.
> 
> AFAIK. A devTLB invalidate descriptor submitted to VT-d should be converted
> to be a PCIe ATC invalidation request without PASID prefix. So it may be
> subjected to the above definition. If so, no need to have a PASID-based
> devTLB invalidation descriptor.
> 

You are correct. The wording in VT-d spec is a bit confusing on saying
that devtlb invalidation descriptor is only for request w/o PASID. 😉