Re: [Intel-gfx] [PATCH v3 12/12] vfio/pci: Report dev_id in VFIO_DEVICE_GET_PCI_HOT_RESET_INFO

From: "Tian, Kevin" <kevin.tian@intel.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
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Subject: Re: [Intel-gfx] [PATCH v3 12/12] vfio/pci: Report dev_id in VFIO_DEVICE_GET_PCI_HOT_RESET_INFO
Date: Tue, 18 Apr 2023 01:28:12 +0000	[thread overview]
Message-ID: <BN9PR11MB52764ECBAE117EE159CECC598C9D9@BN9PR11MB5276.namprd11.prod.outlook.com> (raw)
In-Reply-To: <ZD1MCc6fD+oisjki@nvidia.com>

> From: Jason Gunthorpe
> Sent: Monday, April 17, 2023 9:39 PM
> 
> On Fri, Apr 14, 2023 at 09:11:30AM +0000, Tian, Kevin wrote:
> 
> > The only corner case with this option is when a user mixes group
> > and cdev usages. iirc you mentioned it's a valid usage to be supported.
> > In that case the kernel doesn't have sufficient knowledge to judge
> > 'resettable' as it doesn't know which groups are opened by this user.
> 
> IMHO we don't need to support this combination.
> 
> We can say that to use the hot reset API the user must put all their
> devices into the same iommufd_ctx and cover 100% of the known use
> cases for this.

Make sense.

> 
> There are already other situations, like nesting, that do force users
> to put everything into one iommufd_ctx.
> 
> No reason to make things harder and more complicated.
> 
> I'm coming to the feeling that we should put no-iommu devices in
> iommufd_ctx's as well. They would be an iommufd_access like
> mdevs. That would clean up the complications they cause here.

This certainly simplifies the matter a lot!

> 
> I suppose we should have done that from the beginning - no-iommu is an
> IOMMUFD access, it just uses a crazy /proc based way to learn the
> PFNs. Making it a proper access and making a real VFIO ioctl that
> calls iommufd_access_pin_pages() and returns the DMA mapped addresses
> to userspace would go a long way to making no-iommu work in a logical,
> usable, way.
> 

Yes. This would provide a more reliable/clean way to learn PFNs for
noiommufd case.