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

[Eric Auger]

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.
>
> 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.

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