Re: [Intel-gfx] [PATCH v6 12/24] vfio/pci: Allow passing zero-length fd array in VFIO_DEVICE_PCI_HOT_RESET

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Mar 22, 2023 at 06:36:14AM -0600, Alex Williamson wrote:
> On Wed, 22 Mar 2023 09:27:16 -0300
> Jason Gunthorpe <jgg@nvidia.com> wrote:
> 
> > On Tue, Mar 21, 2023 at 04:47:37PM -0600, Alex Williamson wrote:
> > > On Tue, 21 Mar 2023 19:20:37 -0300
> > > Jason Gunthorpe <jgg@nvidia.com> wrote:
> > >   
> > > > On Tue, Mar 21, 2023 at 03:01:12PM -0600, Alex Williamson wrote:
> > > >   
> > > > > > Though it would be nice if qemu didn't need two implementations so Yi
> > > > > > I'd rather see a new info in this series as well and qemu can just
> > > > > > consistently use dev_id and never bdf in iommufd mode.    
> > > > > 
> > > > > We also need to consider how libvirt determines if QEMU has the kernel
> > > > > support it needs to pass file descriptors.  It'd be a lot cleaner if
> > > > > this aligned with the introduction of vfio cdevs.    
> > > > 
> > > > Yes, that would be much better if it was one package.
> > > > 
> > > > But this is starting to grow and we have so many threads that need to
> > > > progress blocked on this cdev enablement :(
> > > > 
> > > > Could we go forward with the cdev main patches and kconfig it to
> > > > experimental or something while the rest of the parts are completed
> > > > and tested through qemu? ie move the vfio-pci reset enablment to after
> > > > the cdev patches?  
> > > 
> > > We need to be able to guarantee that there cannot be any significant
> > > builds of the kernel with vfio cdev support if our intention is to stage
> > > it for libvirt.  We don't have a global EXPERIMENTAL config option any
> > > more.  Adding new code under BROKEN seems wrong.  Fedora ships with
> > > STAGING enabled.  A sternly worded Kconfig entry is toothless.  What is
> > > the proposed mechanism to make this not look like a big uncompiled code
> > > dump?  Thanks,  
> > 
> > I would suggest a sternly worded kconfig and STAGING.
> > 
> > This isn't such a big issue, we are trying to say that a future
> > released qemu is not required to work on older kernels with a STAGING
> > kconfig mark.
> > 
> > IOW we are saying that qemu release X.0 with production iommufd
> > requires kernel version > x.y and just lightly reflecting this into
> > the kconfig.
> > 
> > qemu should simply not support iommufd if it finds itself on a old
> > kernel.
> 
> Inferring features based on kernel versions doesn't work in a world
> where downstreams backport features to older kernels. 

I don't mean actual kernel versions as a compatability test. I mean it
as documention and an expected "support" window.

ie we are disclaiming support for STAGING kernel as a matter of
documentation, not code.

Jason