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

[Jason Gunthorpe]

On Mon, May 20, 2024 at 04:59:18AM +0000, Tian, Kevin wrote:
> > From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> > Sent: Monday, May 20, 2024 11:33 AM
> > 
> > On 5/20/24 11:24 AM, Tian, Kevin wrote:
> > >> From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> > >> Sent: Sunday, May 19, 2024 10:38 PM
> > >>
> > >> On 2024/5/15 15:43, Tian, Kevin wrote:
> > >>>> From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > >>>> 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@xxxxxxxx/
> > 
> 
> 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