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

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Date:   Fri, 1 Apr 2022 14:20:23 +0800
From:   Yi Liu <yi.l.liu@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        "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>,
        "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 2022/3/29 19:42, Jason Gunthorpe wrote:
> 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.

so the group size can be immutable for specific topology. right? I think 
for non-multi-function devices plugged behind an PCIE bridge which has 
enabled ACS, such devices should have their own groups. Under such topology 
the group size should be 1 constantly. May just enable SVA for such devices.

> 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.

I think this may be a device plugged in a PCIE-to-PCI bridge, and then 
hotplug a device to this bridge. The group size is variable. right? Per my 
understanding, maybe such a bridge cannot support PASID Prefix at all, 
hence no SVA support for such devices.

-- 
Regards,
Yi Liu