Re: [PATCH v2 6/6] vfio: platform: move get/put reset at open/release

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 2015-10-22 at 16:23 +0200, Eric Auger wrote:
> On 10/22/2015 04:10 PM, Arnd Bergmann wrote:
> > On Thursday 22 October 2015 15:26:55 Eric Auger wrote:
> >>>> @@ -181,6 +182,8 @@ static int vfio_platform_open(void *device_data)
> >>>>                 if (ret)
> >>>>                         goto err_irq;
> >>>>  
> >>>> +               vfio_platform_get_reset(vdev);
> >>>> +
> >>>>                 if (vdev->reset)
> >>>>                         vdev->reset(vdev);
> >>>>
> >>>
> >>> This needs some error handling to ensure that the open() fails
> >>> if there is no reset handler.
> >>
> >> Is that really what we want? The code was meant to allow the use case
> >> where the VFIO platform driver would be used without such reset module.
> >>
> >> I think the imperious need for a reset module depends on the device and
> >> more importantly depends on the IOMMU mapping. With QEMU VFIO
> >> integration this is needed because the whole VM memory is IOMMU mapped
> >> but in a simpler user-space driver context, we might live without.
> >>
> >> Any thought?
> > 
> > I would think we need a reset driver for any device that can start DMA,
> > otherwise things can go wrong as soon as you attach it to a different domain
> > while there is ongoing DMA.
> > 
> > Maybe we could just allow devices to be attached without a reset handler,
> > but then disallow DMA on them?
> 
> Well I am tempted to think that most assigned devices will perform DMA
> accesses so to me this somehow comes to the same result, ie disallowing
> functional passthrough for devices not properly/fully integrated.
> 
> Alex/Baptiste, any opinion on this?

We have an IOMMU and the user doesn't get access to the device until the
IOMMU domain is established.  So, ideally yes, we should have a way to
reset the device, but I don't see it as a requirement.  Thanks,

Alex