linux-kernel - Re: [PATCH v2 03/12] iommu: Remove unrecoverable fault data

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Message-ID: <ZNUUYR9WGf475Q4L@ziepe.ca>
Date:   Thu, 10 Aug 2023 13:46:25 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Kevin Tian <kevin.tian@...el.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, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 03/12] iommu: Remove unrecoverable fault data

On Thu, Aug 10, 2023 at 10:27:21AM +0800, Baolu Lu wrote:
> On 2023/8/10 0:59, Jason Gunthorpe wrote:
> > On Thu, Jul 27, 2023 at 01:48:28PM +0800, Lu Baolu wrote:
> > > The unrecoverable fault data is not used anywhere. Remove it to avoid
> > > dead code.
> > > 
> > > Suggested-by: Kevin Tian<kevin.tian@...el.com>
> > > Signed-off-by: Lu Baolu<baolu.lu@...ux.intel.com>
> > > ---
> > >   include/linux/iommu.h | 70 +------------------------------------------
> > >   1 file changed, 1 insertion(+), 69 deletions(-)
> > Do we plan to bring this back in some form? A driver specific fault
> > report via iommufd?
> 
> I can hardly see the possibility.
> 
> The only necessary dma fault messages are the offending address and the
> permissions. With these, the user space device model software knows that
> "a DMA fault was generated when the IOMMU hardware tried to translate
> the offending address with the given permissions".
> 
> And then, the device model software will walk the page table and figure
> out what is missed before injecting the vendor-specific fault messages
> to the VM guest.

Avoiding walking the page table sounds like a pretty big win if we
could manage it by forwarding more event data..

Jason