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

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Date:   Fri, 1 Dec 2023 10:24:27 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Lu Baolu <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 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.

If the userspace wants a fd per domain then it should do that. If it
wants to share fds between domains that should work too.

Jason