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

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Message-ID: <20210401160337.GJ1463678@nvidia.com>
Date:   Thu, 1 Apr 2021 13:03:37 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Liu, Yi L" <yi.l.liu@...el.com>
Cc:     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>,
        Eric Auger <eric.auger@...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 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.

I don't remember any device specific actions in ATS, so what is the
driver supposed to do?

> 4) device driver's fault handler signals an event FD to notify userspace to
>    fetch the information about the page fault. If it's VM case, inject the
>    page fault to VM and let guest to solve it.

If the PASID is set to 'report page fault to userspace' then some
event should come out of /dev/ioasid, or be reported to a linked
eventfd, or whatever.

If the PASID is set to 'SVM' then the fault should be passed to
handle_mm_fault

And so on.

Userspace chooses what happens based on how they configure the PASID
through /dev/ioasid.

Why would a device driver get involved here?

> Eric has sent below series for the page fault reporting for VM with passthru
> device.
> https://lore.kernel.org/kvm/20210223210625.604517-5-eric.auger@redhat.com/

It certainly should not be in vfio pci. Everything using a PASID needs
this infrastructure, VDPA, mdev, PCI, CXL, etc.

Jason