Re: [Xen-devel] [PATCH v3 18/24] xen/passthrough: iommu_deassign_device_dt: By default reassign device to nobody

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 23/02/15 16:47, Jan Beulich wrote:
>>>> On 23.02.15 at 17:37, <julien.grall@xxxxxxxxxx> wrote:
>> On 23/02/15 15:39, Ian Campbell wrote:
>>> On Mon, 2015-02-23 at 10:20 +0000, Jan Beulich wrote:
>>>>>>> On 23.02.15 at 11:10, <julien.grall@xxxxxxxxxx> wrote:
>>>>> Right now, there is no possibility to reset a platform device in the 
>>>>> kernel. There is some discussion about it but nothing more.
>>>
>>> "there is no possibility to reset a platform device" isn't quite true,
>>> there is certainly a theoretical possibility (i.e. it is obviously the
>>> case that a dom0 could be written to do so for at least some devices).
>>
>>> So what happens if such a dom0 arises in the future? I suppose the
>>> intention is that the user would having deassigned from domU and
>>> determined that there kernel has the necessary feature would do some
>>> sort of xl command to assign to dom0?
>>
>> The list of things to do for resetting a device would be:
>>      - Deassign device from DOMU (though it's done during destruction)
>>      - Assign the device to DOM0 and map MMIO
>>      - Reset the device
>>
>> But that not my concern right now. As we agreed, this feature (i.e reset
>> a device) is not in the scope of this series.
>>
>> So assigning the device to DOM0 is not safe at all.
> 
> Kind of confusing - above you state "assign the device to DOM0
> and map MMIO" as a prereq step for resetting the device, yet
> then you state assigning it back to Dom0 is not safe. Which of the
> two do you mean after all?

Ian was asking how would it be possible to reset the device and I gave
him a list of things to do.

Reassigning the device to DOM0 is not safe if there is no reset driver
in the DOM0 kernel. Unlike PCI, there is no generic way to reset a
platform device.

As for now, there is no reset driver at all in DOM0. Not reassign the
device is the better thing to do.

Regards,

-- 
Julien Grall

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