Re: [PATCH v3 12/12] vfio/pci: Report dev_id in VFIO_DEVICE_GET_PCI_HOT_RESET_INFO

[Yi Liu <yi.l.liu@intel.com>]

On 2023/4/8 22:20, Alex Williamson wrote:
> On Sat, 8 Apr 2023 05:07:16 +0000
> "Liu, Yi L" <yi.l.liu@intel.com> wrote:
> 
>>> From: Alex Williamson <alex.williamson@redhat.com>
>>> Sent: Saturday, April 8, 2023 5:07 AM
>>>
>>> On Fri, 7 Apr 2023 15:47:10 +0000
>>> "Liu, Yi L" <yi.l.liu@intel.com> wrote:
>>>    
>>>>> From: Alex Williamson <alex.williamson@redhat.com>
>>>>> Sent: Friday, April 7, 2023 11:14 PM
>>>>>
>>>>> On Fri, 7 Apr 2023 14:04:02 +0000
>>>>> "Liu, Yi L" <yi.l.liu@intel.com> wrote:
>>>>>   
>>>>>>> From: Alex Williamson <alex.williamson@redhat.com>
>>>>>>> Sent: Friday, April 7, 2023 9:52 PM
>>>>>>>
>>>>>>> On Fri, 7 Apr 2023 13:24:25 +0000
>>>>>>> "Liu, Yi L" <yi.l.liu@intel.com> wrote:
>>>>>>>   
>>>>>>>>> From: Alex Williamson <alex.williamson@redhat.com>
>>>>>>>>> Sent: Friday, April 7, 2023 8:04 PM
>>>>>>>>>   
>>>>>>>>>>>>> @@ -791,7 +813,21 @@ static int vfio_pci_fill_devs(struct pci_dev
>>>>> *pdev,
>>>>>>> void
>>>>>>>>>>> *data)
>>>>>>>>>>>>>   	if (!iommu_group)
>>>>>>>>>>>>>   		return -EPERM; /* Cannot reset non-isolated devices
>>> */
>>>>>>>>
>>>>>>>> [1]
>>>>>>>>   
>>>>>>>>>>>>
>>>>>>>>>>>> Hi Alex,
>>>>>>>>>>>>
>>>>>>>>>>>> Is disabling iommu a sane way to test vfio noiommu mode?
>>>>>>>>>>>
>>>>>>>>>>> Yes
>>>>>>>>>>>   
>>>>>>>>>>>> I added intel_iommu=off to disable intel iommu and bind a device to
>>> vfio-
>>>>> pci.
>>>>>>>>>>>> I can see the /dev/vfio/noiommu-0 and /dev/vfio/devices/noiommu-
>>> vfio0.
>>>>>>> Bind
>>>>>>>>>>>> iommufd==-1 can succeed, but failed to get hot reset info due to the
>>>>> above
>>>>>>>>>>>> group check. Reason is that this happens to have some affected
>>> devices,
>>>>> and
>>>>>>>>>>>> these devices have no valid iommu_group (because they are not
>>> bound to
>>>>>>> vfio-
>>>>>>>>> pci
>>>>>>>>>>>> hence nobody allocates noiommu group for them). So when hot reset
>>> info
>>>>>>> loops
>>>>>>>>>>>> such devices, it failed with -EPERM. Is this expected?
>>>>>>>>>>>
>>>>>>>>>>> Hmm, I didn't recall that we put in such a limitation, but given the
>>>>>>>>>>> minimally intrusive approach to no-iommu and the fact that we never
>>>>>>>>>>> defined an invalid group ID to return to the user, it makes sense that
>>>>>>>>>>> we just blocked the ioctl for no-iommu use.  I guess we can do the same
>>>>>>>>>>> for no-iommu cdev.
>>>>>>>>>>
>>>>>>>>>> I just realize a further issue related to this limitation. Remember that we
>>>>>>>>>> may finally compile out the vfio group infrastructure in the future. Say I
>>>>>>>>>> want to test noiommu, I may boot such a kernel with iommu disabled. I
>>> think
>>>>>>>>>> the _INFO ioctl would fail as there is no iommu_group. Does it mean we
>>> will
>>>>>>>>>> not support hot reset for noiommu in future if vfio group infrastructure is
>>>>>>>>>> compiled out?
>>>>>>>>>
>>>>>>>>> We're talking about IOMMU groups, IOMMU groups are always present
>>>>>>>>> regardless of whether we expose a vfio group interface to userspace.
>>>>>>>>> Remember, we create IOMMU groups even in the no-iommu case.  Even
>>> with
>>>>>>>>> pure cdev, there are underlying IOMMU groups that maintain the DMA
>>>>>>>>> ownership.
>>>>>>>>
>>>>>>>> hmmm. As [1], when iommu is disabled, there will be no iommu_group for a
>>>>>>>> given device unless it is registered to VFIO, which a fake group is created.
>>>>>>>> That's why I hit the limitation [1]. When vfio_group is compiled out, then
>>>>>>>> even fake group goes away.
>>>>>>>
>>>>>>> In the vfio group case, [1] can be hit with no-iommu only when there
>>>>>>> are affected devices which are not bound to vfio.
>>>>>>
>>>>>> yes. because vfio would allocate fake group when device is registered to
>>>>>> it.
>>>>>>   
>>>>>>> Why are we not
>>>>>>> allocating an IOMMU group to no-iommu devices when vfio group is
>>>>>>> disabled?  Thanks,
>>>>>>
>>>>>> hmmm. when the vfio group code is configured out. The
>>>>>> vfio_device_set_group() just returns 0 after below patch is
>>>>>> applied and CONFIG_VFIO_GROUP=n. So when there is no
>>>>>> vfio group, the fake group also goes away.
>>>>>>
>>>>>> https://lore.kernel.org/kvm/20230401151833.124749-25-yi.l.liu@intel.com/
>>>>>
>>>>> Is this a fundamental issue or just a problem with the current
>>>>> implementation proposal?  It seems like the latter.  FWIW, I also don't
>>>>> see a taint happening in the cdev path for no-iommu use.  Thanks,
>>>>
>>>> yes. the latter case. The reason I raised it here is to confirm the
>>>> policy on the new group/bdf capability in the DEVICE_GET_INFO. If
>>>> there is no iommu group, perhaps I only need to exclude the new
>>>> group/bdf capability from the cap chain of DEVICE_GET_INFO. is it?
>>>
>>> I think we need to revisit the question of why allocating an IOMMU
>>> group for a no-iommu device is exclusive to the vfio group support.
>>
>> For no-iommu device, the iommu group is a fake group allocated by vfio.
>> is it? And the fake group allocation is part of the vfio group code.
>> It is the vfio_device_set_group() in group.c. If vfio group code is not
>> compiled in, vfio does not allocate fake groups. Detail for this compiling
>> can be found in link [1].
>>
>>> We've already been down the path of trying to report a field that only
>>> exists for devices with certain properties with dev-id.  It doesn't
>>> work well.  I think we've said all along that while the cdev interface
>>> is device based, there are still going to be underlying IOMMU groups
>>> for the user to be aware of, they're just not as much a fundamental
>>> part of the interface.  There should not be a case where a device
>>> doesn't have a group to report.  Thanks,
>>
>> As the patch in link [1] makes vfio group optional, so if compile a kernel
>> with CONFIG_VFIO_GROUP=n, and boot it with iommu disabled, then there is no
>> group to report. Perhaps this is not a typical usage but still a sane usage
>> for noiommu mode as I confirmed with you in this thread. So when it comes,
>> needs to consider what to report for the group field.
>>
>> Perhaps I messed up the discussion by referring to a patch that is part of
>> another series. But I think it should be considered when talking about the
>> group to be reported.
> 
> The question is whether the split that group.c code handles both the
> vfio group AND creation of the IOMMU group in such cases is the correct
> split.  I'm not arguing that the way the code is currently laid out has
> the fake IOMMU group for no-iommu devices created in vfio group
> specific code, but we have a common interface that makes use of IOMMU
> group information for which we don't have an equivalent alternative
> data field to report.

yes. It is needed to ensure _HOT_RESET_INFO workable for noiommu devices.

> We've shown that dev-id doesn't work here because dev-ids only exist
> for devices within the user's IOMMU context.  Also reporting an invalid
> ID of any sort fails to indicate the potential implied ownership.
> Therefore I recognize that if this interface is to report an IOMMU
> group, then the creation of fake IOMMU groups existing only in vfio
> group code would need to be refactored.  Thanks,

yeah, needs to move the iommu group creation back to vfio_main.c. This
would be a prerequisite for [1]

[1] https://lore.kernel.org/kvm/20230401151833.124749-25-yi.l.liu@intel.com/

I'll also try out your suggestion to add a capability like below and link
it in the vfio_device_info cap chain.

#define VFIO_DEVICE_INFO_CAP_PCI_BDF          5

struct vfio_device_info_cap_pci_bdf {
         struct vfio_info_cap_header header;
         __u32   group_id;
         __u16   segment;
         __u8    bus;
         __u8    devfn; /* Use PCI_SLOT/PCI_FUNC */
};

-- 
Regards,
Yi Liu