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

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Message-ID: <7694a4ea-4a08-2296-cd3a-593004de8718@linux.intel.com>
Date:   Fri, 11 Aug 2023 09:15:44 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     baolu.lu@...ux.intel.com, 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 2023/8/11 0:46, Jason Gunthorpe wrote:
> 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..

Fair enough. We can discuss what kind of extra event data could be
included later when we have real code for dma fault forwarding support
in iommufd.

Best regards,
baolu