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

[Liu, Yi L]

> From: Alex Williamson <alex.williamson at redhat.com>
> Sent: Wednesday, April 19, 2023 2:39 AM
> 
> On Tue, 18 Apr 2023 09:57:32 -0300
> Jason Gunthorpe <jgg at nvidia.com> wrote:
> 
> > On Mon, Apr 17, 2023 at 02:06:42PM -0600, Alex Williamson wrote:
> > > On Mon, 17 Apr 2023 16:31:56 -0300
> > > Jason Gunthorpe <jgg at nvidia.com> wrote:
> > >
> > > > On Mon, Apr 17, 2023 at 01:01:40PM -0600, Alex Williamson wrote:
> > > > > Yes, it's not trivial, but Jason is now proposing that we consider
> > > > > mixing groups, cdevs, and multiple iommufd_ctxs as invalid.  I think
> > > > > this means that regardless of which device calls INFO, there's only one
> > > > > answer (assuming same set of devices opened, all cdev, all within same
> > > > > iommufd_ctx).  Based on what I explained about my understanding of INFO2
> > > > > and Jason agreed to, I think the output would be:
> > > > >
> > > > > flags: NOT_RESETABLE | DEV_ID
> > > > > {
> > > > >   { valid devA-id,  devA-BDF },
> > > > >   { valid devC-id,  devC-BDF },
> > > > >   { valid devD-id,  devD-BDF },
> > > > >   { invalid dev-id, devE-BDF },
> > > > > }
> > > > >
> > > > > Here devB gets dropped because the kernel understands that devB is
> > > > > unopened, affected, and owned.  It's therefore not a blocker for
> > > > > hot-reset.
> > > >
> > > > I don't think we want to drop anything because it makes the API
> > > > ill suited for the debugging purpose.
> > > >
> > > > devb should be returned with an invalid dev_id if I understand your
> > > > example. Maybe it should return with -1 as the dev_id instead of 0, to
> > > > make the debugging a bit better.
> > > >
> > > > Userspace should look at only NOT_RESETTABLE to determine if it
> > > > proceeds or not, and it should use the valid dev_id list to iterate
> > > > over the devices it has open to do the config stuff.
> > >
> > > If an affected device is owned, not opened, and not interfering with
> > > the reset, what is it adding to the API to report it for debugging
> > > purposes?
> >
> > It lets it print the entire group of devices, this is the only way
> > something can learn the actual list of all BDFs affected.
> 
> If we do so, userspace must be able to differentiate which devices are
> blocking, which necessitates at least a bi-modal invalid dev-id.
> 
> > dev_id can just return 0, we don't need a complex bitmap. Userspace
> > looks at the flag, if !NOT_RESETABLE then it ignores dev_id=0.
> 
> I'm having trouble with a succinct definition of dev-id == 0, is it "A
> device affected by the hot-reset reset, which does not directly
> contribute to the availability of the hot-reset, ex. an unopened device
> within the same IOMMU group as an opened device (ie. this is not the
> device responsible if hot-reset is unavailable). 

Hide this device in the list looks fine to me. But the calling user should
not do any new device open before finishing hot-reset. Otherwise, user may
miss a device that needs to do pre/post reset. I think this requirement is
acceptable. Is it? 

> Whereas dev-id < 0
> (== -1) is an affected device which prevents hot-reset, ex. an un-owned
> device, device configured within a different iommufd_ctx, or device
> opened outside of the vfio cdev API."  Is that about right?  Thanks,

Do you mean to have separate err-code for the three possibilities? As
the devid is generated by iommufd and it is u32. I'm not sure if we can
have such err-code definition without reserving some ids in iommufd. 

Regards,
Yi Liu