Re: [PATCH 3/4] KVM: PPC: Add support for IOMMU in-kernel handling

From: Alexey Kardashevskiy
Date: Thu Jun 20 2013 - 04:48:30 EST

On 06/20/2013 05:47 PM, Benjamin Herrenschmidt wrote:
> On Thu, 2013-06-20 at 15:28 +1000, David Gibson wrote:
>>> Just out of curiosity - would not get_file() and fput_atomic() on a
>> group's
>>> file* do the right job instead of vfio_group_add_external_user() and
>>> vfio_group_del_external_user()?
>> I was thinking that too. Grabbing a file reference would certainly be
>> the usual way of handling this sort of thing.
> But that wouldn't prevent the group ownership to be returned to
> the kernel or another user would it ?

Holding the file pointer does not let the group->container_users counter go
to zero and this is exactly what vfio_group_add_external_user() and
vfio_group_del_external_user() do. The difference is only in absolute value
- 2 vs. 3.

No change in behaviour whether I use new vfio API or simply hold file* till
KVM closes fd created when IOMMU was connected to LIOBN.

And while this counter is not zero, QEMU cannot take ownership over the group.

I am definitely still missing the bigger picture...

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