Re: [PATCH] iommu/virtio: Advertise IOMMU_CAP_CACHE_COHERENCY

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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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