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

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Message-ID: <20231107175405.GD4634@ziepe.ca>
Date:   Tue, 7 Nov 2023 13:54:05 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.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>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/6] IOMMUFD: Deliver IO page faults to user space

On Tue, Nov 07, 2023 at 08:35:10AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@...pe.ca>
> > Sent: Thursday, November 2, 2023 8:48 PM
> >
> > 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.
> > 
> > Having now looked more closely at the ARM requirements it seems we
> > will need generic events, not just page fault events to have a
> > complete emulation.
> 
> Can you elaborate?

There are many events related to object in guest memory or controlled
by the guest, eg C_BAD_CD and C_BAD_STE. These should be relayed or
the emulation is not working well.

> > > 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 the right way to approach it, and more broadly this shouldn't
> > be an iommufd specific thing. Kernel drivers will also need to create
> > fault capable PAGING iommu domains.
> 
> Are you suggesting a common interface used by both iommufd and
> kernel drivers?

Yes
 
> but I didn't get the last piece. If those domains are created by kernel
> drivers why would they require a uAPI for userspace to specify fault
> capable?

Not to userspace, but a kapi to request a fault capable domain and to
supply the fault handler. Eg:

 iommu_domain_alloc_faultable(dev, handler);

Jason