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

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Apr 18, 2023 at 10:23:55AM +0000, Liu, Yi L wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > 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.
> 
> Do you mean we don't support hot-reset for this combination or we don't
> support user using this combination. I guess the prior one. Right?

Yes

> Ditto. We just fail hot-reset for the multiple iommufds case. Is it?

Yes

> > 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.
> 
> This seems to be an improvement for noiommu mode. It can be done later.
> For now, generating access_id and binding noiommu devices with iommufdctx
> is enough for supporting noiommu hot-reset.

Yes, I'm not sure there is much value in improving no-iommu unless
someone also wants to go in and update dpdk.

At some point we will need to revise dpdk to use iommufd, maybe that
would be a good time to fix this too.

The point is that using an access is actually a logical and sensible
thing to do, no a hack to make hot reset work better.

Jason