RE: [RFC 0/7] VIRTIO-IOMMU/VFIO: Fix host iommu geometry handling for hotplugged devices

[Duan, Zhenzhong]

>-----Original Message-----
>From: Eric Auger <eric.auger@redhat.com>
>Subject: Re: [RFC 0/7] VIRTIO-IOMMU/VFIO: Fix host iommu geometry
>handling for hotplugged devices
>
>Hi Zhenzhong,
>On 1/18/24 08:10, Duan, Zhenzhong wrote:
>> Hi Eric,
>>
>>> -----Original Message-----
>>> From: Eric Auger <eric.auger@redhat.com>
>>> Cc: mst@redhat.com; clg@redhat.com
>>> Subject: [RFC 0/7] VIRTIO-IOMMU/VFIO: Fix host iommu geometry
>handling
>>> for hotplugged devices
>>>
>>> In [1] we attempted to fix a case where a VFIO-PCI device protected
>>> with a virtio-iommu was assigned to an x86 guest. On x86 the physical
>>> IOMMU may have an address width (gaw) of 39 or 48 bits whereas the
>>> virtio-iommu used to expose a 64b address space by default.
>>> Hence the guest was trying to use the full 64b space and we hit
>>> DMA MAP failures. To work around this issue we managed to pass
>>> usable IOVA regions (excluding the out of range space) from VFIO
>>> to the virtio-iommu device. This was made feasible by introducing
>>> a new IOMMU Memory Region callback dubbed
>iommu_set_iova_regions().
>>> This latter gets called when the IOMMU MR is enabled which
>>> causes the vfio_listener_region_add() to be called.
>>>
>>> However with VFIO-PCI hotplug, this technique fails due to the
>>> race between the call to the callback in the add memory listener
>>> and the virtio-iommu probe request. Indeed the probe request gets
>>> called before the attach to the domain. So in that case the usable
>>> regions are communicated after the probe request and fail to be
>>> conveyed to the guest. To be honest the problem was hinted by
>>> Jean-Philippe in [1] and I should have been more careful at
>>> listening to him and testing with hotplug :-(
>> It looks the global virtio_iommu_config.bypass is never cleared in guest.
>> When guest virtio_iommu driver enable IOMMU, should it clear this
>> bypass attribute?
>> If it could be cleared in viommu_probe(), then qemu will call
>> virtio_iommu_set_config() then virtio_iommu_switch_address_space_all()
>> to enable IOMMU MR. Then both coldplugged and hotplugged devices will
>work.
>
>this field is iommu wide while the probe applies on a one device.In
>general I would prefer not to be dependent on the MR enablement. We know
>that the device is likely to be protected and we can collect its
>requirements beforehand.       

Agree that your new patch is cleaner.

>>
>> Intel iommu has a similar bit in register GCMD_REG.TE, when guest
>> intel_iommu driver probe set it, on qemu side,
>vtd_address_space_refresh_all()
>> is called to enable IOMMU MRs.
>interesting.
>
>Would be curious to get Jean Philippe's pov.
>>
>>> For coldplugged device the technique works because we make sure all
>>> the IOMMU MR are enabled once on the machine init done: 94df5b2180
>>> ("virtio-iommu: Fix 64kB host page size VFIO device assignment")
>>> for granule freeze. But I would be keen to get rid of this trick.
>>>
>>> Using an IOMMU MR Ops is unpractical because this relies on the
>IOMMU
>>> MR to have been enabled and the corresponding
>vfio_listener_region_add()
>>> to be executed. Instead this series proposes to replace the usage of this
>>> API by the recently introduced PCIIOMMUOps: ba7d12eb8c  ("hw/pci:
>>> modify
>>> pci_setup_iommu() to set PCIIOMMUOps"). That way, the callback can be
>>> called earlier, once the usable IOVA regions have been collected by
>>> VFIO, without the need for the IOMMU MR to be enabled.
>>>
>>> This looks cleaner. In the short term this may also be used for
>>> passing the page size mask, which would allow to get rid of the
>>> hacky transient IOMMU MR enablement mentionned above.
>>>
>>> [1] [PATCH v4 00/12] VIRTIO-IOMMU/VFIO: Don't assume 64b IOVA
>space
>>>    https://lore.kernel.org/all/20231019134651.842175-1-
>>> eric.auger@redhat.com/
>>>
>>> [2] https://lore.kernel.org/all/20230929161547.GB2957297@myrica/
>>>
>>> Extra Notes:
>>> With that series, the reserved memory regions are communicated on
>time
>>> so that the virtio-iommu probe request grabs them. However this is not
>>> sufficient. In some cases (my case), I still see some DMA MAP failures
>>> and the guest keeps on using IOVA ranges outside the geometry of the
>>> physical IOMMU. This is due to the fact the VFIO-PCI device is in the
>>> same iommu group as the pcie root port. Normally the kernel
>>> iova_reserve_iommu_regions (dma-iommu.c) is supposed to call
>>> reserve_iova()
>>> for each reserved IOVA, which carves them out of the allocator. When
>>> iommu_dma_init_domain() gets called for the hotplugged vfio-pci device
>>> the iova domain is already allocated and set and we don't call
>>> iova_reserve_iommu_regions() again for the vfio-pci device. So its
>>> corresponding reserved regions are not properly taken into account.
>> I suspect there is same issue with coldplugged devices. If those devices
>> are in same group, get iova_reserve_iommu_regions() is only called
>> for first device. But other devices's reserved regions are missed.
>
>Correct
>>
>> Curious how you make passthrough device and pcie root port under same
>> group.
>> When I start a x86 guest with passthrough device, I see passthrough
>> device and pcie root port are in different group.
>>
>> -[0000:00]-+-00.0
>>            +-01.0
>>            +-02.0
>>            +-03.0-[01]----00.0
>>
>> /sys/kernel/iommu_groups/3/devices:
>> 0000:00:03.0
>> /sys/kernel/iommu_groups/7/devices:
>> 0000:01:00.0
>>
>> My qemu cmdline:
>> -device pcie-root-port,id=root0,slot=0
>> -device vfio-pci,host=6f:01.0,id=vfio0,bus=root0
>
>I just replayed the scenario:
>- if you have a coldplugged vfio-pci device, the pci root port and the
>passthroughed device end up in different iommu groups. On my end I use
>ioh3420 but you confirmed that's the same for the generic pcie-root-port
>- however if you hotplug the vfio-pci device that's a different story:
>they end up in the same group. Don't ask me why. I tried with
>both virtio-iommu and intel iommu and I end up with the same topology.
>That looks really weird to me.

That's strange. I tested two vfio devices with ioh3420, one coldplug, the other 
hotplug.
ioh3420 and vfio devices are in same group.

-[0000:00]-+-00.0
           +-01.0
           +-02.0
           +-03.0-[01]----00.0
           +-04.0-[02]----00.0

/sys/kernel/iommu_groups/3/devices:
0000:00:03.0  0000:01:00.0

/sys/kernel/iommu_groups/4/devices:
0000:00:04.0  0000:02:00.0

Thanks
Zhenzhong

>
>I initially thought this was an ACS issue but I am now puzzled.
>
>Thanks!
>
>Eric
>>
>> Thanks
>> Zhenzhong
>>
>>> This is not trivial to fix because theoretically the 1st attached
>>> devices could already have allocated IOVAs within the reserved regions
>>> of the second device. Also we are somehow hijacking the reserved
>>> memory regions to model the geometry of the physical IOMMU so not
>sure
>>> any attempt to fix that upstream will be accepted. At the moment one
>>> solution is to make sure assigned devices end up in singleton group.
>>> Another solution is to work on a different approach where the gaw
>>> can be passed as an option to the virtio-iommu device, similarly at
>>> what is done with intel iommu.
>>>
>>> This series can be found at:
>>> https://github.com/eauger/qemu/tree/hotplug-resv-rfc
>>>
>>>
>>> Eric Auger (7):
>>>  hw/pci: Introduce PCIIOMMUOps::set_host_iova_regions
>>>  hw/pci: Introduce pci_device_iommu_bus
>>>  vfio/pci: Pass the usable IOVA ranges through PCIIOMMUOps
>>>  virtio-iommu: Implement PCIIOMMUOps set_host_resv_regions
>>>  virtio-iommu: Remove the implementation of iommu_set_iova_ranges
>>>  hw/vfio: Remove memory_region_iommu_set_iova_ranges() call
>>>  memory: Remove IOMMU MR iommu_set_iova_range API
>>>
>>> include/exec/memory.h    |  32 -------
>>> include/hw/pci/pci.h     |  16 ++++
>>> hw/pci/pci.c             |  16 ++++
>>> hw/vfio/common.c         |  10 --
>>> hw/vfio/pci.c            |  27 ++++++
>>> hw/virtio/virtio-iommu.c | 201 ++++++++++++++++++++-------------------
>>> system/memory.c          |  13 ---
>>> 7 files changed, 160 insertions(+), 155 deletions(-)
>>>
>>> --
>>> 2.41.0