linux-kernel - RE: [PATCH RFC v2 02/11] iommu: Add iommu_group_singleton

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Message-ID: <BN9PR11MB52761EC0B212CB66878076568C1F9@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Wed, 30 Mar 2022 06:55:00 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        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>
CC:     Eric Auger <eric.auger@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH RFC v2 02/11] iommu: Add iommu_group_singleton_lockdown()

> From: Lu Baolu <baolu.lu@...ux.intel.com>
> Sent: Wednesday, March 30, 2022 1:00 PM
> >
> > btw I'm not sure whether this is what SVA requires. IIRC the problem with
> > SVA is because PASID TLP prefix is not counted in PCI packet routing thus
> > a DMA target address with PASID might be treated as P2P if the address
> > falls into the MMIO BAR of other devices in the group. This is why the
> > original code needs to strictly apply SVA in a group containing a single
> > device, instead of a group attached by a single driver, unless we want to
> > reserve those MMIO ranges in CPU VA space.
> 
> You are right. But I don't think the IOMMU core is able to guarantee
> above in a platform/device-agnostic way. Or any suggestions?
> 
> I guess this should be somewhat off-loaded to the device driver which
> knows details of the device. The device driver should know this and
> guarantee it before calling
> iommu_dev_enable_feature(dev, IOMMU_DEV_FEAT_SVA).

How would the device driver know whether SVA requests from a
device might be mis-interpreted as p2p by upstreaming ports?

> 
> This patch itself just replaces the existing
> "iommu_group_device_count(group) != 1" logic with a new one based on the
> group ownership logistics. The former is obviously not friendly to
> device hot joined afterward.
> 

IMHO this replacement changes the semantics and device hotplug is
something that we must deal with...

Thanks
Kevin