Re: Plan for /dev/ioasid RFC v2

[Lu Baolu]

On 2021/6/24 12:26, David Gibson wrote:
> On Fri, Jun 18, 2021 at 04:57:40PM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > Sent: Friday, June 18, 2021 8:20 AM
> > >
> > > On Thu, Jun 17, 2021 at 03:14:52PM -0600, Alex Williamson wrote:
> > >
> > > > I've referred to this as a limitation of type1, that we can't put
> > > > devices within the same group into different address spaces, such as
> > > > behind separate vRoot-Ports in a vIOMMU config, but really, who cares?
> > > > As isolation support improves we see fewer multi-device groups, this
> > > > scenario becomes the exception.  Buy better hardware to use the devices
> > > > independently.
> > >
> > > This is basically my thinking too, but my conclusion is that we should
> > > not continue to make groups central to the API.
> > >
> > > As I've explained to David this is actually causing functional
> > > problems and mess - and I don't see a clean way to keep groups central
> > > but still have the device in control of what is happening. We need
> > > this device <-> iommu connection to be direct to robustly model all
> > > the things that are in the RFC.
> > >
> > > To keep groups central someone needs to sketch out how to solve
> > > today's mdev SW page table and mdev PASID issues in a clean
> > > way. Device centric is my suggestion on how to make it clean, but I
> > > haven't heard an alternative??
> > >
> > > So, I view the purpose of this discussion to scope out what a
> > > device-centric world looks like and then if we can securely fit in the
> > > legacy non-isolated world on top of that clean future oriented
> > > API. Then decide if it is work worth doing or not.
> > >
> > > To my mind it looks like it is not so bad, granted not every detail is
> > > clear, and no code has be sketched, but I don't see a big scary
> > > blocker emerging. An extra ioctl or two, some special logic that
> > > activates for >1 device groups that looks a lot like VFIO's current
> > > logic..
> > >
> > > At some level I would be perfectly fine if we made the group FD part
> > > of the API for >1 device groups - except that complexifies every user
> > > space implementation to deal with that. It doesn't feel like a good
> > > trade off.
> >
> > Would it be an acceptable tradeoff by leaving >1 device groups
> > supported only via legacy VFIO (which is anyway kept for backward
> > compatibility), if we think such scenario is being deprecated over
> > time (thus little value to add new features on it)? Then all new
> > sub-systems including vdpa and new vfio only support singleton
> > device group via /dev/iommu...
>
> The case that worries me here is if you *thought* you had 1 device
> groups, but then discover a hardware bug which means two things aren't
> as isolated as you thought they were.  What do you do then?

Normally a hardware bug/quirk is identified during boot. For above case,
iommu core should put these two devices in a same iommu_group during
iommu_probe_device() phase. Any runtime hardware bug should be reported
to the OS through various methods so that the device could be quiet
and isolated. I don't think two devices could be in different groups
initially and then be moved to a single one.

Best regards,
baolu