linux-kernel - Re: [PATCH v2 0/6] IOMMUFD: Deliver IO page faults to user space

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Date:   Fri, 8 Dec 2023 09:43:17 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     Kevin Tian <kevin.tian@...el.com>, Joerg Roedel <joro@...tes.org>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Nicolin Chen <nicolinc@...dia.com>,
        Yi Liu <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        iommu@...ts.linux.dev, linux-kselftest@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/6] IOMMUFD: Deliver IO page faults to user space

On Fri, Dec 08, 2023 at 01:57:26PM +0800, Baolu Lu wrote:
> On 12/1/23 10:24 PM, Jason Gunthorpe wrote:
> > On Thu, Oct 26, 2023 at 10:49:24AM +0800, Lu Baolu wrote:
> > > Hi folks,
> > > 
> > > This series implements the functionality of delivering IO page faults to
> > > user space through the IOMMUFD framework for nested translation. Nested
> > > translation is a hardware feature that supports two-stage translation
> > > tables for IOMMU. The second-stage translation table is managed by the
> > > host VMM, while the first-stage translation table is owned by user
> > > space. This allows user space to control the IOMMU mappings for its
> > > devices.
> > > 
> > > When an IO page fault occurs on the first-stage translation table, the
> > > IOMMU hardware can deliver the page fault to user space through the
> > > IOMMUFD framework. User space can then handle the page fault and respond
> > > to the device top-down through the IOMMUFD. This allows user space to
> > > implement its own IO page fault handling policies.
> > > 
> > > User space indicates its capability of handling IO page faults by
> > > setting the IOMMU_HWPT_ALLOC_IOPF_CAPABLE flag when allocating a
> > > hardware page table (HWPT). IOMMUFD will then set up its infrastructure
> > > for page fault delivery. On a successful return of HWPT allocation, the
> > > user can retrieve and respond to page faults by reading and writing to
> > > the file descriptor (FD) returned in out_fault_fd.
> > 
> > This is probably backwards, userspace should allocate the FD with a
> > dedicated ioctl and provide it during domain allocation.
> 
> Introducing a dedicated fault FD for fault handling seems promising. It
> decouples the fault handling from any specific domain. I suppose we need
> different fault fd for recoverable faults (a.k.a. IO page fault) and
> unrecoverable faults. Do I understand you correctly?

I haven't thought that far ahead :) Once you have a generic fault FD
concept it can be sliced in different ways. If there is a technical
need to seperate recoverable/unrecoverable then the FD flavour should
be specified during FD creation. Otherwise just let userspace do
whatever it wants.

Jason