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

[Jean-Philippe Brucker]

Hi Eric,

On Tue, Jan 23, 2024 at 07:15:54PM +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 64b with arm virt.

Doesn't Arm have the same problem?  The TTB0 page tables limit what can be
mapped to 48-bit, or 52-bit when SMMU_IDR5.VAX==1 and granule is 64kB.
A Linux host driver could configure smaller VA sizes:
* SMMUv2 limits the VA to SMMU_IDR2.UBS (upstream bus size) which
  can go as low as 32-bit (I'm assuming we don't care about 32-bit hosts).
* SMMUv3 currently limits the VA to CONFIG_ARM64_VA_BITS, which
  could be as low as 36 bits (but realistically 39, since 36 depends on
  16kB pages and CONFIG_EXPERT).

But 64-bit definitely can't work for VFIO, and I suppose isn't useful for
virtual devices, so maybe 39 is also a reasonable default on Arm.

Thanks,
Jean

> This replaces the
> previous default value of 64b. So we need to introduce a compat
> for pc_q35 machines older than 9.0 to behave similarly.