[PATCH kernel 14/15] vfio/spapr_tce: Export container API for external users
Alexey Kardashevskiy
aik at ozlabs.ru
Mon Aug 29 16:35:15 AEST 2016
On 18/08/16 10:22, Alexey Kardashevskiy wrote:
> On 17/08/16 13:17, David Gibson wrote:
>> On Fri, Aug 12, 2016 at 09:22:01AM -0600, Alex Williamson wrote:
>>> On Fri, 12 Aug 2016 15:46:01 +1000
>>> David Gibson <david at gibson.dropbear.id.au> wrote:
>>>
>>>> On Wed, Aug 10, 2016 at 10:46:30AM -0600, Alex Williamson wrote:
>>>>> On Wed, 10 Aug 2016 15:37:17 +1000
>>>>> Alexey Kardashevskiy <aik at ozlabs.ru> wrote:
>>>>>
>>>>>> On 09/08/16 22:16, Alex Williamson wrote:
>>>>>>> On Tue, 9 Aug 2016 15:19:39 +1000
>>>>>>> Alexey Kardashevskiy <aik at ozlabs.ru> wrote:
>>>>>>>
>>>>>>>> On 09/08/16 02:43, Alex Williamson wrote:
>>>>>>>>> On Wed, 3 Aug 2016 18:40:55 +1000
>>>>>>>>> Alexey Kardashevskiy <aik at ozlabs.ru> wrote:
>>>>>>>>>
>>>>>>>>>> This exports helpers which are needed to keep a VFIO container in
>>>>>>>>>> memory while there are external users such as KVM.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>
>>>>>>>>>> ---
>>>>>>>>>> drivers/vfio/vfio.c | 30 ++++++++++++++++++++++++++++++
>>>>>>>>>> drivers/vfio/vfio_iommu_spapr_tce.c | 16 +++++++++++++++-
>>>>>>>>>> include/linux/vfio.h | 6 ++++++
>>>>>>>>>> 3 files changed, 51 insertions(+), 1 deletion(-)
>>>>>>>>>>
>>>>>>>>>> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
>>>>>>>>>> index d1d70e0..baf6a9c 100644
>>>>>>>>>> --- a/drivers/vfio/vfio.c
>>>>>>>>>> +++ b/drivers/vfio/vfio.c
>>>>>>>>>> @@ -1729,6 +1729,36 @@ long vfio_external_check_extension(struct vfio_group *group, unsigned long arg)
>>>>>>>>>> EXPORT_SYMBOL_GPL(vfio_external_check_extension);
>>>>>>>>>>
>>>>>>>>>> /**
>>>>>>>>>> + * External user API for containers, exported by symbols to be linked
>>>>>>>>>> + * dynamically.
>>>>>>>>>> + *
>>>>>>>>>> + */
>>>>>>>>>> +struct vfio_container *vfio_container_get_ext(struct file *filep)
>>>>>>>>>> +{
>>>>>>>>>> + struct vfio_container *container = filep->private_data;
>>>>>>>>>> +
>>>>>>>>>> + if (filep->f_op != &vfio_fops)
>>>>>>>>>> + return ERR_PTR(-EINVAL);
>>>>>>>>>> +
>>>>>>>>>> + vfio_container_get(container);
>>>>>>>>>> +
>>>>>>>>>> + return container;
>>>>>>>>>> +}
>>>>>>>>>> +EXPORT_SYMBOL_GPL(vfio_container_get_ext);
>>>>>>>>>> +
>>>>>>>>>> +void vfio_container_put_ext(struct vfio_container *container)
>>>>>>>>>> +{
>>>>>>>>>> + vfio_container_put(container);
>>>>>>>>>> +}
>>>>>>>>>> +EXPORT_SYMBOL_GPL(vfio_container_put_ext);
>>>>>>>>>> +
>>>>>>>>>> +void *vfio_container_get_iommu_data_ext(struct vfio_container *container)
>>>>>>>>>> +{
>>>>>>>>>> + return container->iommu_data;
>>>>>>>>>> +}
>>>>>>>>>> +EXPORT_SYMBOL_GPL(vfio_container_get_iommu_data_ext);
>>>>>>>>>> +
>>>>>>>>>> +/**
>>>>>>>>>> * Sub-module support
>>>>>>>>>> */
>>>>>>>>>> /*
>>>>>>>>>> diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
>>>>>>>>>> index 3594ad3..fceea3d 100644
>>>>>>>>>> --- a/drivers/vfio/vfio_iommu_spapr_tce.c
>>>>>>>>>> +++ b/drivers/vfio/vfio_iommu_spapr_tce.c
>>>>>>>>>> @@ -1331,6 +1331,21 @@ const struct vfio_iommu_driver_ops tce_iommu_driver_ops = {
>>>>>>>>>> .detach_group = tce_iommu_detach_group,
>>>>>>>>>> };
>>>>>>>>>>
>>>>>>>>>> +struct iommu_table *vfio_container_spapr_tce_table_get_ext(void *iommu_data,
>>>>>>>>>> + u64 offset)
>>>>>>>>>> +{
>>>>>>>>>> + struct tce_container *container = iommu_data;
>>>>>>>>>> + struct iommu_table *tbl = NULL;
>>>>>>>>>> +
>>>>>>>>>> + if (tce_iommu_find_table(container, offset, &tbl) < 0)
>>>>>>>>>> + return NULL;
>>>>>>>>>> +
>>>>>>>>>> + iommu_table_get(tbl);
>>>>>>>>>> +
>>>>>>>>>> + return tbl;
>>>>>>>>>> +}
>>>>>>>>>> +EXPORT_SYMBOL_GPL(vfio_container_spapr_tce_table_get_ext);
>>>>>>>>>> +
>>>>>>>>>> static int __init tce_iommu_init(void)
>>>>>>>>>> {
>>>>>>>>>> return vfio_register_iommu_driver(&tce_iommu_driver_ops);
>>>>>>>>>> @@ -1348,4 +1363,3 @@ MODULE_VERSION(DRIVER_VERSION);
>>>>>>>>>> MODULE_LICENSE("GPL v2");
>>>>>>>>>> MODULE_AUTHOR(DRIVER_AUTHOR);
>>>>>>>>>> MODULE_DESCRIPTION(DRIVER_DESC);
>>>>>>>>>> -
>>>>>>>>>> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
>>>>>>>>>> index 0ecae0b..1c2138a 100644
>>>>>>>>>> --- a/include/linux/vfio.h
>>>>>>>>>> +++ b/include/linux/vfio.h
>>>>>>>>>> @@ -91,6 +91,12 @@ extern void vfio_group_put_external_user(struct vfio_group *group);
>>>>>>>>>> extern int vfio_external_user_iommu_id(struct vfio_group *group);
>>>>>>>>>> extern long vfio_external_check_extension(struct vfio_group *group,
>>>>>>>>>> unsigned long arg);
>>>>>>>>>> +extern struct vfio_container *vfio_container_get_ext(struct file *filep);
>>>>>>>>>> +extern void vfio_container_put_ext(struct vfio_container *container);
>>>>>>>>>> +extern void *vfio_container_get_iommu_data_ext(
>>>>>>>>>> + struct vfio_container *container);
>>>>>>>>>> +extern struct iommu_table *vfio_container_spapr_tce_table_get_ext(
>>>>>>>>>> + void *iommu_data, u64 offset);
>>>>>>>>>>
>>>>>>>>>> /*
>>>>>>>>>> * Sub-module helpers
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> I think you need to take a closer look of the lifecycle of a container,
>>>>>>>>> having a reference means the container itself won't go away, but only
>>>>>>>>> having a group set within that container holds the actual IOMMU
>>>>>>>>> references. container->iommu_data is going to be NULL once the
>>>>>>>>> groups are lost. Thanks,
>>>>>>>>
>>>>>>>>
>>>>>>>> Container owns the iommu tables and this is what I care about here, groups
>>>>>>>> attached or not - this is handled separately via IOMMU group list in a
>>>>>>>> specific iommu_table struct, these groups get detached from iommu_table
>>>>>>>> when they are removed from a container.
>>>>>>>
>>>>>>> The container doesn't own anything, the container is privileged by the
>>>>>>> groups being attached to it. When groups are closed, they detach from
>>>>>>> the container and once the container group list is empty the iommu
>>>>>>> backend is released and iommu_data is NULL. A container reference
>>>>>>> doesn't give you what you're looking for. It implies nothing about the
>>>>>>> iommu backend.
>>>>>>
>>>>>>
>>>>>> Well. Backend is a part of a container and since a backend owns tables, a
>>>>>> container owns them too.
>>>>>
>>>>> The IOMMU backend is accessed through the container, but that backend
>>>>> is privileged by the groups it contains. Once those groups are gone,
>>>>> the IOMMU backend is released, regardless of whatever reference you
>>>>> have to the container itself such as you're attempting to do here. In
>>>>> that sense, the container does not own those tables.
>>>>
>>>> So, the thing is that what KVM fundamentally needs is a handle on the
>>>> container. KVM is essentially modelling the DMA address space of a
>>>> single guest bus, and the container is what's attached to that.
>>>>
>>>> The first part of the problem is that KVM wants to basically invoke
>>>> vfio_dma_map() operations without bouncing via qemu. Because
>>>> vfio_dma_map() works on the container level, that's the handle that
>>>> KVM needs to hold.
>>>>
>>>> The second part of the problem is that in order to reduce overhead
>>>> further, we want to operate in real mode, which means bypassing most
>>>> of the usual VFIO structure and going directly(ish) from the KVM
>>>> hcall emulation to the IOMMU backend behind VFIO. This complicates
>>>> matters a fair bit. Because it is, explicitly, a performance hack,
>>>> some degree of ugliness is probably inevitable.
>>>>
>>>> Alexey - actually implementing this in two stages might make this
>>>> clearer. The first stage wouldn't allow real mode, and would call
>>>> through the same vfio_dma_map() path as qemu calls through now. The
>>>> second stage would then put in place the necessary hacks to add real
>>>> mode support.
>>>>
>>>>>> The problem I am trying to solve here is when KVM may release the
>>>>>> iommu_table objects.
>>>>>>
>>>>>> "Set" ioctl() to KVM-spapr-tce-table (or KVM itself, does not really
>>>>>> matter) makes a link between KVM-spapr-tce-table and container and KVM can
>>>>>> start using tables (with referencing them).
>>>>>>
>>>>>> First I tried adding an "unset" ioctl to KVM-spapr-tce-table, called it
>>>>>> from region_del() and this works if QEMU removes a window. However if QEMU
>>>>>> removes a vfio-pci device, region_del() is not called and KVM does not get
>>>>>> notified that it can release the iommu_table's because the
>>>>>> KVM-spapr-tce-table remains alive and does not get destroyed (as it is
>>>>>> still used by emulated devices or other containers).
>>>>>>
>>>>>> So it was suggested that we could do such "unset" somehow later assuming,
>>>>>> for example, on every "set" I could check if some of currently attached
>>>>>> containers are no more used - and this is where being able to know if there
>>>>>> is no backend helps - KVM remembers a container pointer and can check this
>>>>>> via vfio_container_get_iommu_data_ext().
>>>>>>
>>>>>> The other option would be changing vfio_container_get_ext() to take a
>>>>>> callback+opaque which container would call when it destroys iommu_data.
>>>>>> This looks more intrusive and not very intuitive how to make it right -
>>>>>> container would have to keep track of all registered external users and
>>>>>> vfio_container_put_ext() would have to pass the same callback+opaque to
>>>>>> unregister the exact external user.
>>>>>
>>>>> I'm not in favor of anything resembling the code above or extensions
>>>>> beyond it, the container is the wrong place to do this.
>>>>>
>>>>>> Or I could store container file* in KVM. Then iommu_data would never be
>>>>>> released until KVM-spapr-tce-table is destroyed.
>>>>>
>>>>> See above, holding a file pointer to the container doesn't do squat.
>>>>> The groups that are held by the container empower the IOMMU backend,
>>>>> references to the container itself don't matter. Those references will
>>>>> not maintain the IOMMU data.
>>>>>
>>>>>> Recreating KVM-spapr-tce-table on every vfio-pci hotunplug (closing its fd
>>>>>> would "unset" container from KVM-spapr-tce-table) is not an option as there
>>>>>> still may be devices using this KVM-spapr-tce-table.
>>>>>>
>>>>>> What obvious and nice solution am I missing here? Thanks.
>>>>>
>>>>> The interactions with the IOMMU backend that seem relevant are
>>>>> vfio_iommu_drivers_ops.{detach_group,release}. The kvm-vfio pseudo
>>>>> device is also used to tell kvm about groups as they come and go and
>>>>> has a way to check extensions, and thus properties of the IOMMU
>>>>> backend. All of these are available for your {ab}use. Thanks,
>>>>
>>>> So, Alexey started trying to do this via the KVM-VFIO device, but it's
>>>> a really bad fit. As noted above, fundamentally it's a container we
>>>> need to attach to the kvm-spapr-tce-table object, since what that
>>>> represents is a guest bus DMA address space, and by definition all the
>>>> groups in a container must have the same DMA address space.
>>>
>>> That's all fine and good, but the point remains that a reference to the
>>> container is no assurance of the iommu state. The iommu state is
>>> maintained by the user and the groups attached to the container. If
>>> the groups are removed, your container reference no long has any iommu
>>> backing and iommu_data is worthless. The user can do this as well by
>>> un-setting the iommu. I understand what you're trying to do, it's just
>>> wrong. Thanks,
>>
>> I'm trying to figure out how to do this right, and it's not at all
>> obvious. The container may be wrong, but that doesn't have the
>> KVM-VFIO device any more useful. Attempting to do this at the group
>> level is at least as wrong for the reasons I've mentioned elsewhere.
>>
>
> I could create a new fd, one per iommu_table, the fd would reference the
> iommu_table (not touching an iommu_table_group or a container), VFIO SPAPR
> TCE backend would return it in VFIO_IOMMU_SPAPR_TCE_CREATE (ioctl which
> creates windows) or I could add VFIO_IOMMU_SPAPR_TCE_GET_FD_BY_OFFSET; then
> I'd pass this new fd to the KVM or KVM-spapr-tce-table to hook them up. To
> release the reference, KVM-spapr-tce-table would have "unset" ioctl()
> or/and on every "set" I would look if all attached tables have at least one
> iommu_table_group attached, if none - release the table.
>
> This would make no change to generic VFIO code and very little change in
> SPAPR TCE backend. Would that be acceptable or it is horrible again? Thanks.
Ping?
--
Alexey
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