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

[Alex Williamson]

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).  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,

Alex