Re: [PATCH v6 5/5] vfio/type1: Use mdev bus iommu_ops for IOMMU callbacks

[Lu Baolu]

Hi Alex,

On 10/31/20 5:06 AM, Alex Williamson wrote:
> On Fri, 30 Oct 2020 06:16:28 +0000
> "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
>
> > > From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > > Sent: Friday, October 30, 2020 12:58 PM
> > >
> > > With the IOMMU driver registering iommu_ops for the mdev_bus, the
> > > IOMMU
> > > operations on an mdev could be done in the same way as any normal device
> > > (for example, PCI/PCIe). There's no need to distinguish an mdev from
> > > others for iommu operations. Remove the unnecessary code.
> >
> > This is really a nice cleanup as the output of this change! :)
>
> It's easy to remove a bunch of code when the result is breaking
> everyone else.  Please share with me how SR-IOV backed mdevs continue
> to work on AMD platforms, or how they might work on ARM platforms, when
> siov_iommu_ops (VT-d only) becomes the one and only provider of
> iommu_ops on the mdev bus.  Hard NAK on this series.  Thanks,

I focused too much on a feature and forgot about university. I should
apologize for this. Sorry about it!

Back to the original intention of this series. The aux domain was
allocated in vfio/mdev, but it's also needed by the vDCM component of a
device driver for mediated callbacks. Currently vfio/mdev or iommu core
has no support for this.

We had a proposal when we first did aux-domain support. But was not
discussed since there was no consumer at that time.

https://lore.kernel.org/linux-iommu/20181105073408.21815-7-baolu.lu@xxxxxxxxxxxxxxx/

Does it look good to you? I can send patches of such solution for
discussion if you think it's a right way.

Extending the iommu core for subdevice passthrough support sounds an
interesting topic, but it will take much time before we reach a
consensus. It sounds a good topic for the next year's LPC/MC :-).

Best regards,
baolu