linux-kernel - Re: [PATCH v4 04/14] PCI/P2PDMA: Clear ACS P2P flags for all devices behind switches

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Message-ID: <f0d25e16-1201-1fc5-334f-4205abc95500@redhat.com>
Date:   Wed, 9 May 2018 11:47:31 -0400
From:   Don Dutile <ddutile@...hat.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Stephen Bates <sbates@...thlin.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH v4 04/14] PCI/P2PDMA: Clear ACS P2P flags for all devices
 behind switches

On 05/08/2018 08:01 PM, Alex Williamson wrote:
> On Tue, 8 May 2018 19:06:17 -0400
> Don Dutile <ddutile@...hat.com> wrote:
>> On 05/08/2018 05:27 PM, Stephen  Bates wrote:
>>> As I understand it VMs need to know because VFIO passes IOMMU
>>> grouping up into the VMs. So if a IOMMU grouping changes the VM's
>>> view of its PCIe topology changes. I think we even have to be
>>> cognizant of the fact the OS running on the VM may not even support
>>> hot-plug of PCI devices.
>> Alex:
>> Really? IOMMU groups are created by the kernel, so don't know how
>> they would be passed into the VMs, unless indirectly via  PCI(e)
>> layout. At best, twiddling w/ACS enablement (emulation) would cause
>> VMs to see different IOMMU groups, but again, VMs are not the
>> security point/level, the host/HV's are.
> 
> Correct, the VM has no concept of the host's IOMMU groups, only the
> hypervisor knows about the groups, but really only to the extent of
> which device belongs to which group and whether the group is viable.
> Any runtime change to grouping though would require DMA mapping
> updates, which I don't see how we can reasonably do with drivers,
> vfio-pci or native host drivers, bound to the affected devices.  Thanks,
> 
> Alex
> 
A change in iommu groups would/could require a device remove/add cycle to get an updated DMA-mapping (yet-another-overused-term: iommu 'domain').