linux-kernel - RE: [PATCH RFC 10/11] iommu: Make IOPF handling framework generic

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Date:   Tue, 22 Mar 2022 01:00:08 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>
CC:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        "Robin Murphy" <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Eric Auger <eric.auger@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH RFC 10/11] iommu: Make IOPF handling framework generic

> From: Jean-Philippe Brucker <jean-philippe@...aro.org>
> Sent: Monday, March 21, 2022 7:42 PM
> 
> Hi Kevin,
> 
> On Mon, Mar 21, 2022 at 08:09:36AM +0000, Tian, Kevin wrote:
> > > From: Lu Baolu <baolu.lu@...ux.intel.com>
> > > Sent: Sunday, March 20, 2022 2:40 PM
> > >
> > > The existing IOPF handling framework only handles the I/O page faults for
> > > SVA. Ginven that we are able to link iommu domain with each I/O page
> fault,
> > > we can now make the I/O page fault handling framework more general
> for
> > > more types of page faults.
> >
> > "make ... generic" in subject line is kind of confusing. Reading this patch I
> > think you really meant changing from per-device fault handling to per-
> domain
> > fault handling. This is more accurate in concept since the fault is caused by
> > the domain page table. 😊
> 
> I tend to disagree with that last part. The fault is caused by a specific
> device accessing shared page tables. We should keep that device
> information throughout the fault handling, so that we can report it to the
> driver when things go wrong. A process can have multiple threads bound to
> different devices, they share the same mm so if the driver wanted to
> signal a misbehaving thread, similarly to a SEGV on the CPU side, it would
> need the device information to precisely report it to userspace.
> 

iommu driver can include the device information in the fault data. But
in concept the IOPF should be reported per domain.

and I agree with Jason that at most we can send SEGV to the entire thread
group since there is no way to associate a DMA back to a thread which 
initiates the DMA.

Thanks
Kevin