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

Robin Murphy
2016-11-07T19:22:19Z

Hi Sricharan,

On 04/11/16 15:16, Sricharan wrote:
> Hi Robin,
>
>>>> Yikes, on second look, that definitely shouldn't be happening.
>>>> Everything below is probably the resulting fallout.
>>>
>>> [ 40.206703] vfio-pci 0000:08:00.0: Failed to setup iommu ops
>>>
>>> I think the above print which says "failed to setup iommu_ops"
>>> because the call ops->add_device failed in of_pci_iommu_configure
>>> is the reason for the failure, in my case i simply do not get this even with
>>> your scripts. ops->add_device succeeds in the rebind as well. So still
>>> checking what could be happening in your case.
>>
>> I was looking at your code base from [1].The ops->add_device
>> callback from of_pci_iommu_configure on the rebind is the
>> one which is causing the failure. But not able to spot out
>>from code which point is causing the failure. It would be very helpful
>> if i can know which is the return value from the add_device callback
>> or point inside add_device callback which fails in your setup.
>>
>>
>> [1] git://linux-arm.org/linux-rm iommu/misc
>
> With little more try, i saw an issue where i had an failure
> similar to what you reported. The issue happens when multiple
> devices fall in to same group due to matching sids. I ended up
> doing a fix like below and it would be nice to verify if it is the same
> that we are seeing in your setup and if the fix makes a difference ?
>
> From: Sricharan R <sricharan-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>
> Date: Fri, 4 Nov 2016 20:28:49 +0530
> Subject: [PATCH] iommu/arm-smmu: Fix group's reference counting
>
> 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)

> Signed-off-by: Sricharan R <sricharan-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>
> ---
> drivers/iommu/arm-smmu.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
> index 71ce4b6..a1d0b3c 100644
> --- a/drivers/iommu/arm-smmu.c
> +++ b/drivers/iommu/arm-smmu.c
> @@ -1516,8 +1516,10 @@ static struct iommu_group *arm_smmu_device_group(struct device *dev)
> group = smmu->s2crs[idx].group;
> }
>
> - if (group)
> + if (group) {
> + iommu_group_get_by_id(iommu_group_id(group));
> return group;

This might as well just be inline, i.e.:

return iommu_group_get_by_id(iommu_group_id(group));

It's a shame we have to go all round the houses when we have the group
right there, but this is probably the most expedient fix. I guess we can
extend the API with some sort of iommu_group_get(group) overload in
future if we really want to.

Robin.

> + }
>
> if (dev_is_pci(dev))
> group = pci_device_group(dev);
>