linux-kernel - RE: [PATCH v10 04/12] iommu: Add attach/detach_dev

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Date:   Fri, 29 Jul 2022 04:22:01 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>,
        Jason Gunthorpe <jgg@...dia.com>
CC:     Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        "Robin Murphy" <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Vinod Koul <vkoul@...nel.org>,
        Eric Auger <eric.auger@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Zhangfei Gao" <zhangfei.gao@...aro.org>,
        "Zhu, Tony" <tony.zhu@...el.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Jean-Philippe Brucker" <jean-philippe@...aro.org>
Subject: RE: [PATCH v10 04/12] iommu: Add attach/detach_dev_pasid iommu
 interface

> From: Baolu Lu <baolu.lu@...ux.intel.com>
> Sent: Friday, July 29, 2022 11:21 AM
> 
> On 2022/7/29 10:56, Tian, Kevin wrote:
> >> +static bool iommu_group_device_pasid_viable(struct iommu_group
> *group,
> >> +					    struct device *dev)
> >> +{
> >> +	int count;
> >> +
> >> +	count = iommu_group_device_count(group);
> >> +	if (count != 1)
> >> +		return false;
> >> +
> >> +	/*
> >> +	 * Block PASID attachment in cases where the PCI fabric is
> >> +	 * routing based on address. PCI/ACS disables that.
> >> +	 */
> >> +	if (dev_is_pci(dev))
> >> +		return pci_acs_path_enabled(to_pci_dev(dev), NULL,
> >> +					    REQ_ACS_FLAGS);
> > I think we are leaning toward doing above check in pci_enable_pasid().
> > Then no singleton check inside iommu core.
> 
> The iommu grouping also considers other things, like PCI alias. There
> are many calls of pci_add_dma_alias() in drivers/pci/quirks.c.
> Therefore, I believe that pci_acs_path_enabled() returning true doesn't
> guarantees a singleton group.

Is there an actual problem of sharing PASID table between aliasing RIDs?
As long as ACS is enabled the device is isolated from other devices
in the fabric. DMA aliases don't change that fact and there is no p2p
between aliasing RIDs.

> 
> >
> > Presumably similar check can be done in DT/ACPI path of enabling pasid?
> >
> 
> I can't find the pasid (or anything similar) enabling interfaces for
> DT or ACPI. They are device specific?
> 

Looks only PCI PASID is supported so far. both in Intel/ARM/AMD
drivers. If other buses will support PASID one day, then ACS-equivalent
can be also checked in their PASID enabling APIs.