[PATCH kernel 14/15] vfio/spapr_tce: Export container API for external users
Alexey Kardashevskiy
aik at ozlabs.ru
Wed Sep 21 16:56:52 AEST 2016
On 07/09/16 19:09, Alexey Kardashevskiy wrote:
> On 29/08/16 23:27, David Gibson wrote:
>> On Mon, Aug 29, 2016 at 04:35:15PM +1000, Alexey Kardashevskiy wrote:
>>> 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?
>>
>> I'm still in Toronto after KVM Forum. I had a detailed discussion
>> about this with Alex W, which I'll write up once I get back.
>>
>> The short version is that Alex more-or-less convinced me that we do
>> need to go back to doing this with an interface based on linking
>> groups to LIOBNs. That leads to an interface that's kind of weird and
>> has some fairly counter-intuitive properties, but in the end it works
>> out better than doing it with containers.
>>
>
>
>
>
> Soooo? :)
When can I expect a full version of how to do this in-kernel thingy? Thanks.
--
Alexey
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