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SubjectRE: [PATCH v5 02/12] iommu/vt-d: Manage scalalble mode PASID tables
Date
Hi Joerg,

> From: Joerg Roedel [mailto:joro@8bytes.org]
> Sent: Monday, December 3, 2018 5:44 AM
> To: Lu Baolu <baolu.lu@linux.intel.com>
> Subject: Re: [PATCH v5 02/12] iommu/vt-d: Manage scalalble mode PASID tables
>
> Hi Baolu,
>
> On Wed, Nov 28, 2018 at 11:54:39AM +0800, Lu Baolu wrote:
> > @@ -2482,12 +2482,13 @@ static struct dmar_domain
> *dmar_insert_one_dev_info(struct intel_iommu *iommu,
> > if (dev)
> > dev->archdata.iommu = info;
> >
> > - if (dev && dev_is_pci(dev) && info->pasid_supported) {
> > + /* PASID table is mandatory for a PCI device in scalable mode. */
> > + if (dev && dev_is_pci(dev) && sm_supported(iommu)) {
>
> This will also allocate a PASID table if the device does not support
> PASIDs, right? Will the table not be used in that case or will the
> device just use the fallback PASID? Isn't it better in that case to have
> no PASID table?

We need to allocate the PASID table in scalable mode, the reason is as below:
In VT-d scalable mode, all address translation is done in PASID-granularity.
For requests-with-PASID, the address translation would be subjected to the
PASID entry specified by the PASID value in the DMA request. However, for
requests-without-PASID, there is no PASID in the DMA request. To fulfil
the translation logic, we've introduced RID2PASID field in sm-context-entry
in VT-d 3.o spec. So that such DMA requests would be subjected to the pasid
entry specified by the PASID value in the RID2PASID field of sm-context-entry.

So for a device without PASID support, we need to at least to have a PASID
entry so that its DMA request (without pasid) can be translated. Thus a PASID
table is needed for such devices.

>
> > @@ -143,18 +143,20 @@ int intel_pasid_alloc_table(struct device *dev)
> > return -ENOMEM;
> > INIT_LIST_HEAD(&pasid_table->dev);
> >
> > - size = sizeof(struct pasid_entry);
> > - count = min_t(int, pci_max_pasids(to_pci_dev(dev)), intel_pasid_max_id);
> > - order = get_order(size * count);
> > + if (info->pasid_supported)
> > + max_pasid = min_t(int, pci_max_pasids(to_pci_dev(dev)),
> > + intel_pasid_max_id);
> > +
> > + size = max_pasid >> (PASID_PDE_SHIFT - 3);
> > + order = size ? get_order(size) : 0;
> > pages = alloc_pages_node(info->iommu->node,
> > - GFP_ATOMIC | __GFP_ZERO,
> > - order);
> > + GFP_ATOMIC | __GFP_ZERO, order);
>
> This is a simple data structure allocation path, does it need
> GFP_ATOMIC?

will leave it to Baolu.

>
> Joerg

Thanks,
Yi Liu

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