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

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Message-ID: <20220329114259.GB1716663@nvidia.com>
Date:   Tue, 29 Mar 2022 08:42:59 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        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>,
        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()

On Tue, Mar 29, 2022 at 08:42:13AM +0000, Tian, Kevin wrote:

> 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.

I think it is not such a good idea to mix up group with this test

Here you want to say that all TLPs from the RID route to the host
bridge - ie ACS is on/etc. This is subtly different from a group with
a single device. Specifically it is an immutable property of the
fabric and doesn't change after hot plug events.

ie if we have a singleton group that doesn't have ACS and someone
hotplugs in another device on a bridge, then our SVA is completely
broken and we get data corruption.

Testing the group size is inherently the wrong test to make.

Jason