linux-kernel - Re: [PATCH v5 4/9] iommufd: Add fault and response message definitions

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Message-ID: <04288162-e5fd-48f3-bb60-a41b4ed2c244@linux.intel.com>
Date: Mon, 20 May 2024 11:33:23 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: "Tian, Kevin" <kevin.tian@...el.com>, Jason Gunthorpe <jgg@...pe.ca>,
 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>,
 Joel Granados <j.granados@...sung.com>
Cc: baolu.lu@...ux.intel.com, "iommu@...ts.linux.dev"
 <iommu@...ts.linux.dev>,
 "virtualization@...ts.linux-foundation.org"
 <virtualization@...ts.linux-foundation.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 4/9] iommufd: Add fault and response message
 definitions

On 5/20/24 11:24 AM, Tian, Kevin wrote:
>> From: Baolu Lu <baolu.lu@...ux.intel.com>
>> Sent: Sunday, May 19, 2024 10:38 PM
>>
>> On 2024/5/15 15:43, Tian, Kevin wrote:
>>>> From: Lu Baolu <baolu.lu@...ux.intel.com>
>>>> Sent: Tuesday, April 30, 2024 10:57 PM
>>>>
>>>> iommu_hwpt_pgfaults represent fault messages that the userspace can
>>>> retrieve. Multiple iommu_hwpt_pgfaults might be put in an iopf group,
>>>> with the IOMMU_PGFAULT_FLAGS_LAST_PAGE flag set only for the last
>>>> iommu_hwpt_pgfault.
>>>
>>> Do you envision extending the same structure to report unrecoverable
>>> fault in the future?
>>
>> I am not envisioning extending this to report unrecoverable faults in
>> the future. The unrecoverable faults are not always related to a hwpt,
>> and therefore it's more suitable to route them through a viommu object
>> which is under discussion in Nicolin's series.
> 
> OK, I'll take a look at that series when reaching it in my TODO list. 😊
> 
>>>> + * @length: a hint of how much data the requestor is expecting to fetch.
>> For
>>>> + *          example, if the PRI initiator knows it is going to do a 10MB
>>>> + *          transfer, it could fill in 10MB and the OS could pre-fault in
>>>> + *          10MB of IOVA. It's default to 0 if there's no such hint.
>>>
>>> This is not clear to me and I don't remember PCIe spec defines such
>>> mechanism.
>>
>> This came up in a previous discussion. While it's not currently part of
> 
> Can you provide a link to that discussion?

https://lore.kernel.org/linux-iommu/20240322170410.GH66976@ziepe.ca/

> 
>> the PCI specification and may not be in the future, we'd like to add
>> this mechanism for potential future advanced device features as it
>> offers significant optimization benefits.
>>
> 
> We design uAPI for real usages. It's a bit weird to introduce a format
> for unknown future features w/o an actual user to demonstrate its
> correctness.

Best regards,
baolu