Re: [RFC] /dev/ioasid uAPI proposal

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Jun 02, 2021 at 04:52:02PM +0800, Jason Wang wrote:
> 
> 在 2021/6/2 上午4:28, Jason Gunthorpe 写道:
> > > I summarized five opens here, about:
> > > 
> > > 1)  Finalizing the name to replace /dev/ioasid;
> > > 2)  Whether one device is allowed to bind to multiple IOASID fd's;
> > > 3)  Carry device information in invalidation/fault reporting uAPI;
> > > 4)  What should/could be specified when allocating an IOASID;
> > > 5)  The protocol between vfio group and kvm;
> > > 
> > > For 1), two alternative names are mentioned: /dev/iommu and
> > > /dev/ioas. I don't have a strong preference and would like to hear
> > > votes from all stakeholders. /dev/iommu is slightly better imho for
> > > two reasons. First, per AMD's presentation in last KVM forum they
> > > implement vIOMMU in hardware thus need to support user-managed
> > > domains. An iommu uAPI notation might make more sense moving
> > > forward. Second, it makes later uAPI naming easier as 'IOASID' can
> > > be always put as an object, e.g. IOMMU_ALLOC_IOASID instead of
> > > IOASID_ALLOC_IOASID.:)
> > I think two years ago I suggested /dev/iommu and it didn't go very far
> > at the time.
> 
> 
> It looks to me using "/dev/iommu" excludes the possibility of implementing
> IOASID in a device specific way (e.g through the co-operation with device
> MMU + platform IOMMU)?

This is intended to be the 'drivers/iommu' subsystem though. I don't
want to see pluggabilit here beyoned what drivers/iommu is providing.

If someone wants to do a something complicated through this interface
then they need to make a drivers/iommu implementation.

Or they need to use the mdev-esque "SW TABLE" mode when their driver
attaches to the interface.

If this is good enough or not for a specific device is an entirely
other question though

> What's more, ATS spec doesn't forbid the device #PF to be reported via a
> device specific way.

And if this is done then a kernel function indicating page fault
should be triggered on the ioasid handle that the device has. It is
still drivers/iommu functionality

Jason