RE: [PATCH V3 0/8] IOMMU probe deferral support

Sricharan
2016-11-24T16:10:58Z

Hi Robin,

<snip..>

>
>>>>>> iommu_group_get_for_dev which gets called in the add_device
>>>>>> callback, increases the reference count of the iommu_group,
>>>>>> so we do an iommu_group_put after that. iommu_group_get_for_dev
>>>>>> inturn calls device_group callback and in the case of arm-smmu
>>>>>> we call generic_device_group/pci_device_group which takes
>>>>>> care of increasing the group's reference. But when we return
>>>>>> an already existing group(when multiple devices have same group)
>>>>>> the reference is not incremented, resulting in issues when the
>>>>>> remove_device callback for the devices is invoked.
>>>>>> Fixing the same here.
>>>>>
>>>>> Bah, yes, this does look like my fault - after flip-flopping between
>>>>> about 3 different ways to keep refcounts for the S2CR entries, none of
>>>>> which would quite work, I ripped it all out but apparently still got
>>>>> things wrong, oh well. Thanks for figuring it out.
>>>>>
>>>>> On the probe-deferral angle, whilst it's useful to have uncovered this
>>>>> bug, I don't think we should actually be calling remove_device() from
>>>>> DMA teardown. I think it's preferable from a user perspective if group
>>>>> numbering remains stable, rather than changing depending on the order in
>>>>> which they unbind/rebind VFIO drivers. I'm really keen to try and get
>>>>> this in shape for 4.10, so I've taken the liberty of hacking up my own
>>>>> branch (iommu/defer) based on v3 - would you mind taking a look at the
>>>>> two "iommu/of:" commits to see what you think? (Ignore the PCI changes
>>>>> to your later patches - that was an experiment which didn't really work out)
>>>>
>>>> Ok, will take a look at this now and respond more on this.
>>>>
>>> Sorry for the delayed response on this. I was OOO for the last few days.
>>> So i tested this branch and it worked fine. I tested it with a pci device
>>> for both normal and deferred probe cases. The of/iommu patches
>>> are the cleanup/preparation patches and it looks fine. One thing is without
>>> calling the remove_device callback, the resources like (smes for exmaple)
>>> and the group association of the device all remain allocated. That does not
>>> feel correct, given that the associated device does not exist. So to
>>> understand that, what happens with VFIO in this case which makes the
>>> group renumbering/rebinding a problem ?
>>>
>>
>> Would it be ok if i post a V4 based on your branch above ?
>
>Sure, as long as none of the hacks slip through :) - I've just pushed
>out a mild rework based on Lorenzo's v9, which I hope shouldn't break
>anything for you.
>

Ok sure, i will test and just the post out the stuff from your branch then
mostly by tomorrow.

>Having thought a bit more about the add/remove thing, I'm inclined to
>agree that the group numbering itself may not be that big an issue in
>practice - sure, it could break my little script, but it looks like QEMU
>and such work with the device ID rather than the group number directly,
>so might not even notice. However, the fact remains that the callbacks
>are intended to handle a device being added to/removed from its bus, and
>will continue to do so on other platforms, so I don't like the idea of
>introducing needlessly different behaviour. If you unbind a driver, the
>stream IDs and everything don't stop existing at the hardware level; the
>struct device to which the in-kernel data belongs still exists and
>doesn't stop being associated with its bus. There's no good reason for
>freeing SMEs that we'll only reallocate again (inadequately-specced
>hardware with not enough SMRs/contexts is not a *good* reason), and

ok, so SMRs/contexts was the reason i was adding the remove_dev
callback, but if thats not good enough then there was no other
intention.

>there are also some strong arguments against letting any stream IDs the
>kernel knows about go back to bypass after a driver has been bound - by

ok, but not sure why is this so ?

>keeping groups around as expected that's something we can implement
>quite easily without having to completely lock down bypass for stream
>IDs the kernel *doesn't* know about.
>

So do you mean in this case to keep the unbound device's group/context bank
to bypass rather than resetting the streamids ?

Regards,
Sricharan