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

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Message-ID: <BN9PR11MB5276A68C9DCDA201826714018CE92@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Mon, 20 May 2024 03:24:26 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Baolu Lu <baolu.lu@...ux.intel.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: "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

> 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?

> 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.