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

Eric Auger
2015-10-22T14:23:56Z

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?

Thanks

Eric

>
> Arnd
>