Re: [PATCH v1 15/22] Add iommufd configure option

[Jason Gunthorpe]

On Wed, Sep 20, 2023 at 02:19:42PM +0200, Cédric Le Goater wrote:
> On 9/20/23 05:42, Duan, Zhenzhong wrote:
> > 
> > 
> > > -----Original Message-----
> > > From: Cédric Le Goater <clg@redhat.com>
> > > Sent: Wednesday, September 20, 2023 1:08 AM
> > > Subject: Re: [PATCH v1 15/22] Add iommufd configure option
> > > 
> > > On 8/30/23 12:37, Zhenzhong Duan wrote:
> > > > This adds "--enable-iommufd/--disable-iommufd" to enable or disable
> > > > iommufd support, enabled by default.
> > > 
> > > Why would someone want to disable support at compile time ? It might
> > 
> > For those users who only want to support legacy container feature?
> > Let me know if you still prefer to drop this patch, I'm fine with that.
> 
> I think it is too early.
> 
> > > have been useful for dev but now QEMU should self-adjust at runtime
> > > depending only on the host capabilities AFAIUI. Am I missing something ?
> > 
> > IOMMUFD doesn't support all features of legacy container, so QEMU
> > doesn't self-adjust at runtime by checking if host supports IOMMUFD.
> > We need to specify it explicitly to use IOMMUFD as below:
> > 
> >      -object iommufd,id=iommufd0
> >      -device vfio-pci,host=0000:02:00.0,iommufd=iommufd0
> 
> OK. I am not sure this is the correct interface yet. At first glance,
> I wouldn't introduce a new object for a simple backend depending on a
> kernel interface. I would tend to prefer a "iommu-something" property
> of the vfio-pci device with string values: "legacy", "iommufd", "default"
> and define the various interfaces (the ops you proposed) for each
> depending on the user preference and the capabilities of the host and
> possibly the device.

I think the idea came from Alex? The major point is to be able to have
libvirt open /dev/iommufd and FD pass it into qemu and then share that
single FD across all VFIOs. qemu will typically not be able to
self-open /dev/iommufd as it is root-only.

So the object is not exactly for the backend, the object is for the
file descriptor.

Adding a legacy/iommufd option to the vfio-pci device string doesn't
address these needs.

Jason