Re: [PATCH v3 0/3] VIRTIO-IOMMU: Introduce an aw-bits option

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Feb 08, 2024 at 11:10:16AM +0100, Eric Auger wrote:
> In [1] and [2] we attempted to fix a case where a VFIO-PCI device
> protected with a virtio-iommu is 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 exposes a 64b input address space by default.
> Hence the guest may try to use the full 64b space and DMA MAP
> failures may be encountered. To work around this issue we endeavoured
> to pass usable host IOVA regions (excluding the out of range space) from
> VFIO to the virtio-iommu device so that the virtio-iommu driver can
> query those latter during the probe request and let the guest iommu
> kernel subsystem carve them out. 
> 
> However if there are several devices in the same iommu group,
> only the reserved regions of the first one are taken into
> account by the iommu subsystem of the guest. This generally
> works on baremetal because devices are not going to
> expose different reserved regions. However in our case, this
> may prevent from taking into account the host iommu geometry.
> 
> So the simplest solution to this problem looks to introduce an
> input address width option, aw-bits, which matches what is
> done on the intel-iommu. By default, from now on it is set
> to 39 bits with pc_q35 and 48 with arm virt. This replaces the
> previous default value of 64b. So we need to introduce a compat
> for machines older than 9.0 to behave similarly. We use
> hw_compat_8_2 to acheive that goal.
> 
> Outstanding series [2] remains useful to let resv regions beeing
> communicated on time before the probe request.
> 
> [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/
>     - This is merged -
> 
> [2] [RFC 0/7] VIRTIO-IOMMU/VFIO: Fix host iommu geometry handling for hotplugged devices
>     https://lore.kernel.org/all/20240117080414.316890-1-eric.auger@redhat.com/
>     - This is pending for review on the ML -
> 
> This series can be found at:
> https://github.com/eauger/qemu/tree/virtio-iommu-aw-bits-v3
> previous
> https://github.com/eauger/qemu/tree/virtio-iommu-aw-bits-v2
> 
> Applied on top of [3]
> [PATCH v2] virtio-iommu: Use qemu_real_host_page_mask as default page_size_mask
> https://lore.kernel.org/all/20240117132039.332273-1-eric.auger@redhat.com/

So, I applied this without that patch until we agree whether there are
compat issues in that one. Seems to work without or did I miss anything?


> History:
> v2 -> v3:
> - Collected Zhenzhong and Cédric's R-b + Yanghang's T-b
> - use &error_abort instead of NULL error handle
>   on object_property_get_uint() call (Cédric)
> - use VTD_HOST_AW_39BIT (Cédric)
> 
> v1 -> v2
> - Limit aw to 48b on ARM
> - Check aw is within [32,64]
> - Use hw_compat_8_2
> 
> 
> Eric Auger (3):
>   virtio-iommu: Add an option to define the input range width
>   virtio-iommu: Trace domain range limits as unsigned int
>   hw: Set virtio-iommu aw-bits default value on pc_q35 and arm virt
> 
>  include/hw/virtio/virtio-iommu.h | 1 +
>  hw/arm/virt.c                    | 6 ++++++
>  hw/core/machine.c                | 5 ++++-
>  hw/i386/pc.c                     | 6 ++++++
>  hw/virtio/virtio-iommu.c         | 7 ++++++-
>  hw/virtio/trace-events           | 2 +-
>  6 files changed, 24 insertions(+), 3 deletions(-)
> 
> -- 
> 2.41.0