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

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Message-ID: <51C2C1CC.9000003@ozlabs.ru>
Date:	Thu, 20 Jun 2013 18:48:12 +1000
From:	Alexey Kardashevskiy <aik@...abs.ru>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
CC:	David Gibson <david@...son.dropbear.id.au>,
	Alex Williamson <alex.williamson@...hat.com>,
	Alexander Graf <agraf@...e.de>, linuxppc-dev@...ts.ozlabs.org,
	Paul Mackerras <paulus@...ba.org>,
	"kvm@...r.kernel.org mailing list" <kvm@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>,
	kvm-ppc@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>,
	Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH 3/4] KVM: PPC: Add support for IOMMU in-kernel handling

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...


-- 
Alexey
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