On 2022-07-14 14:00, Jean-Philippe Brucker wrote:
On Thu, Jul 14, 2022 at 01:01:37PM +0100, Robin Murphy wrote:
On 2022-07-14 12:11, Jean-Philippe Brucker wrote:
Fix virtio-iommu interaction with VFIO, as VFIO now requires
IOMMU_CAP_CACHE_COHERENCY. virtio-iommu does not support non-cacheable
mappings, and always expects to be called with IOMMU_CACHE.
Can we know this is actually true though? What if the virtio-iommu
implementation is backed by something other than VFIO, and the underlying
hardware isn't coherent? AFAICS the spec doesn't disallow that.
Right, I should add a note about that. If someone does actually want to
support non-coherent device, I assume we'll add a per-device property, a
'non-cacheable' mapping flag, and IOMMU_CAP_CACHE_COHERENCY will hold.
I'm also planning to add a check on (IOMMU_CACHE && !IOMMU_NOEXEC) in
viommu_map(), but not as a fix.
But what about all the I/O-coherent PL330s? :P (IIRC you can actually
make a Juno do that with S2CR.MTCFG hacks...)
In the meantime we do need to restore VFIO support under virtio-iommu,
since userspace still expects that to work, and the existing use-cases are
coherent devices.
Yeah, I'm not necessarily against adding this as a horrible bodge for
now - the reality is that people using VFIO must be doing it on coherent
systems or it wouldn't be working properly anyway - as long as we all
agree that that's what it is.
Next cycle I'll be sending the follow-up patches to bring
device_iommu_capable() to its final form (hoping the outstanding VDPA
patch lands in the meantime), at which point we get to sort-of-fix the
SMMU drivers[1], and can do something similar here too. I guess the main
question for virtio-iommu is whether it needs to be described/negotiated
in the protocol itself, or can be reliably described by other standard
firmware properties (with maybe just a spec not to clarify that
coherency must be consistent).
Cheers,
Robin.
[1]
https://gitlab.arm.com/linux-arm/linux-rm/-/commit/d8256bf48c8606cbaa6f0815696c2a6dbb72f1b0
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