linux-kernel - Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

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Date:   Thu, 15 Apr 2021 20:07:32 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Auger Eric <eric.auger@...hat.com>
Cc:     "Liu, Yi L" <yi.l.liu@...el.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>
Subject: Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and
 allocation APIs

On Thu, Apr 15, 2021 at 03:11:19PM +0200, Auger Eric wrote:
> Hi Jason,
> 
> On 4/1/21 6:03 PM, Jason Gunthorpe wrote:
> > On Thu, Apr 01, 2021 at 02:08:17PM +0000, Liu, Yi L wrote:
> > 
> >> DMA page faults are delivered to root-complex via page request message and
> >> it is per-device according to PCIe spec. Page request handling flow is:
> >>
> >> 1) iommu driver receives a page request from device
> >> 2) iommu driver parses the page request message. Get the RID,PASID, faulted
> >>    page and requested permissions etc.
> >> 3) iommu driver triggers fault handler registered by device driver with
> >>    iommu_report_device_fault()
> > 
> > This seems confused.
> > 
> > The PASID should define how to handle the page fault, not the driver.
> 
> In my series I don't use PASID at all. I am just enabling nested stage
> and the guest uses a single context. I don't allocate any user PASID at
> any point.
> 
> When there is a fault at physical level (a stage 1 fault that concerns
> the guest), this latter needs to be reported and injected into the
> guest. The vfio pci driver registers a fault handler to the iommu layer
> and in that fault handler it fills a circ bugger and triggers an eventfd
> that is listened to by the VFIO-PCI QEMU device. this latter retrives
> the faault from the mmapped circ buffer, it knowns which vIOMMU it is
> attached to, and passes the fault to the vIOMMU.
> Then the vIOMMU triggers and IRQ in the guest.
> 
> We are reusing the existing concepts from VFIO, region, IRQ to do that.
> 
> For that use case, would you also use /dev/ioasid?

/dev/ioasid could do all the things you described vfio-pci as doing,
it can even do them the same way you just described.

Stated another way, do you plan to duplicate all of this code someday
for vfio-cxl? What about for vfio-platform? ARM SMMU can be hooked to
platform devices, right?

I feel what you guys are struggling with is some choice in the iommu
kernel APIs that cause the events to be delivered to the pci_device
owner, not the PASID owner.

That feels solvable.

Jason