Re: [PATCH v2 0/4] Make the iommu driver no-snoop block feature consistent

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Apr 07, 2022 at 08:27:05PM +0100, Robin Murphy wrote:
> On 2022-04-07 20:08, Jason Gunthorpe wrote:
> > On Thu, Apr 07, 2022 at 07:02:03PM +0100, Robin Murphy wrote:
> > > On 2022-04-07 18:43, Jason Gunthorpe wrote:
> > > > On Thu, Apr 07, 2022 at 06:03:37PM +0100, Robin Murphy wrote:
> > > > > At a glance, this all looks about the right shape to me now, thanks!
> > > > 
> > > > Thanks!
> > > > 
> > > > > Ideally I'd hope patch #4 could go straight to device_iommu_capable() from
> > > > > my Thunderbolt series, but we can figure that out in a couple of weeks once
> > > > 
> > > > Yes, this does helps that because now the only iommu_capable call is
> > > > in a context where a device is available :)
> > > 
> > > Derp, of course I have *two* VFIO patches waiting, the other one touching
> > > the iommu_capable() calls (there's still IOMMU_CAP_INTR_REMAP, which, much
> > > as I hate it and would love to boot all that stuff over to
> > > drivers/irqchip,
> > 
> > Oh me too...
> > 
> > > it's not in my way so I'm leaving it be for now). I'll have to rebase that
> > > anyway, so merging this as-is is absolutely fine!
> > 
> > This might help your effort - after this series and this below there
> > are no 'bus' users of iommu_capable left at all.
> 
> Thanks, but I still need a device for the iommu_domain_alloc() as well, so
> at that point the interrupt check is OK to stay where it is. 

It is a simple enough change that could avoid introducing the
device_iommu_capable() at all perhaps.

> I figured out a locking strategy per my original idea that seems
> pretty clean, it just needs vfio_group_viable() to go away first:

I think this should be more like:

  	        struct vfio_device *vdev;

		mutex_lock(&group->device_lock);
		vdev = list_first_entry(group->device_list, struct vfio_device, group_next);
		ret = driver->ops->attach_group(data, group->iommu_group,
						group->type,
						vdev->dev);
		mutex_unlock(&group->device_lock);

Then don't do the iommu_group_for_each_dev() at all.

The problem with iommu_group_for_each_dev() is that it may select a
struct device that does not have a vfio_device bound to it, so we
would be using a random struct device that is not protected by any
VFIO device_driver.

However, this creates an oddball situation where the vfio_device and
it's struct device could become unplugged from the system while the
domain that the struct device spawned continues to exist and remains
attached to other devices in the same group. ie the iommu driver has
to be careful not to retain the struct device input..

I suppose that is inevitable to have sharing of domains across
devices, so the iommu drivers will have to accommodate this.

Jason