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

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Message-ID: <BN9PR11MB5276EEE89AB66C0EFB6D4DA88CE92@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Mon, 20 May 2024 04:59:18 +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: Monday, May 20, 2024 11:33 AM
> 
> 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
> >>>>
> >>>> + * @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/
> 

We can always extend uAPI for new usages, e.g. having a new flag
bit to indicate the additional filed for carrying the number of pages.
But requiring the user to handle non-zero length now (though trivial)
is unnecessary burden.

Do we want the response message to also carry a length field i.e.
allowing the user to partially fix the fault?