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

[Cédric Le Goater]

On 9/20/23 14:51, Jason Gunthorpe wrote:
> 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 

ok.

> and then share that single FD across all VFIOs. 

I will ask Alex to help me catch up on the topic.

> qemu will typically not be able to
> self-open /dev/iommufd as it is root-only.

I don't understand, we open multiple fds to KVM devices. This is the same.

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

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

I agree.

Thanks,

C.

> Jason