linux-kernel - Re: [PATCH 03/10] vfio: add external user support

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Message-ID: <51EE47DE.5020002@ozlabs.ru>
Date:	Tue, 23 Jul 2013 19:07:42 +1000
From:	Alexey Kardashevskiy <aik@...abs.ru>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
CC:	Alex Williamson <alex.williamson@...hat.com>,
	linuxppc-dev@...ts.ozlabs.org,
	David Gibson <david@...son.dropbear.id.au>,
	Paul Mackerras <paulus@...ba.org>,
	Alexander Graf <agraf@...e.de>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm-ppc@...r.kernel.org
Subject: Re: [PATCH 03/10] vfio: add external user support

On 07/23/2013 12:23 PM, Alex Williamson wrote:
> On Tue, 2013-07-16 at 10:53 +1000, Alexey Kardashevskiy wrote:
>> VFIO is designed to be used via ioctls on file descriptors
>> returned by VFIO.
>>
>> However in some situations support for an external user is required.
>> The first user is KVM on PPC64 (SPAPR TCE protocol) which is going to
>> use the existing VFIO groups for exclusive access in real/virtual mode
>> on a host to avoid passing map/unmap requests to the user space which
>> would made things pretty slow.
>>
>> The protocol includes:
>>
>> 1. do normal VFIO init operation:
>> 	- opening a new container;
>> 	- attaching group(s) to it;
>> 	- setting an IOMMU driver for a container.
>> When IOMMU is set for a container, all groups in it are
>> considered ready to use by an external user.
>>
>> 2. User space passes a group fd to an external user.
>> The external user calls vfio_group_get_external_user()
>> to verify that:
>> 	- the group is initialized;
>> 	- IOMMU is set for it.
>> If both checks passed, vfio_group_get_external_user()
>> increments the container user counter to prevent
>> the VFIO group from disposal before KVM exits.
>>
>> 3. The external user calls vfio_external_user_iommu_id()
>> to know an IOMMU ID. PPC64 KVM uses it to link logical bus
>> number (LIOBN) with IOMMU ID.
>>
>> 4. When the external KVM finishes, it calls
>> vfio_group_put_external_user() to release the VFIO group.
>> This call decrements the container user counter.
>> Everything gets released.
>>
>> The "vfio: Limit group opens" patch is also required for the consistency.
>>
>> Signed-off-by: Alexey Kardashevskiy <aik@...abs.ru>
> 
> This looks fine to me.  Is the plan to add this through the ppc tree
> again?  Thanks,


Nope, better to add this through your tree. And faster for sure :) Thanks!



-- 
Alexey
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