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

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Message-ID: <20240524141506.GN69273@ziepe.ca>
Date: Fri, 24 May 2024 11:15:06 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: "Tian, Kevin" <kevin.tian@...el.com>
Cc: Baolu Lu <baolu.lu@...ux.intel.com>, 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>,
	"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 Mon, May 20, 2024 at 04:59:18AM +0000, Tian, Kevin wrote:
> > 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.

It is tricky to extend this stuff since it comes out in read().. We'd
have to have userspace negotiate a new format most likely.

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

No, the device will discover this when it gets another fault  :)

Jason