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

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Message-ID: <5628F17C.9030907@linaro.org>
Date:	Thu, 22 Oct 2015 16:23:56 +0200
From:	Eric Auger <eric.auger@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	eric.auger@...com, alex.williamson@...hat.com,
	b.reynal@...tualopensystems.com,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
	kvm@...r.kernel.org, christoffer.dall@...aro.org,
	linux-kernel@...r.kernel.org, patches@...aro.org
Subject: Re: [PATCH v2 6/6] vfio: platform: move get/put reset at open/release

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
> 

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