On 2022-08-25 16:46, Jean-Philippe Brucker wrote:

Commit e8ae0e140c05 ("vfio: Require that devices support DMA cache coherence") requires IOMMU drivers to advertise IOMMU_CAP_CACHE_COHERENCY, in order to be used by VFIO. Since VFIO does not provide to userspace the ability to maintain coherency through cache invalidations, it requires hardware coherency. Advertise the capability in order to restore VFIO support.

The meaning of IOMMU_CAP_CACHE_COHERENCY also changed from "IOMMU can enforce cache coherent DMA transactions" to "IOMMU_CACHE is supported".

Argh! Massive apologies, I've been totally overlooking that detail and forgetting that we ended up splitting out the dedicated enforce_cache_coherency op... I do need reminding sometimes :)

While virtio-iommu cannot enforce coherency (of PCIe no-snoop transactions), it does support IOMMU_CACHE.

We can distinguish different cases of non-coherent DMA:

(1) When accesses from a hardware endpoint are not coherent. The host would describe such a device using firmware methods ('dma-coherent' in device-tree, '_CCA' in ACPI), since they are also needed without a vIOMMU. In this case mappings are created without IOMMU_CACHE. virtio-iommu doesn't need any additional support. It sends the same requests as for coherent devices.

(2) When the physical IOMMU supports non-cacheable mappings. Supporting those would require a new feature in virtio-iommu, new PROBE request property and MAP flags. Device drivers would use a new API to discover this since it depends on the architecture and the physical IOMMU.

(3) When the hardware supports PCIe no-snoop. It is possible for assigned PCIe devices to issue no-snoop transactions, and the virtio-iommu specification is lacking any mention of this.

 Arm platforms don't necessarily support no-snoop, and those that do
 cannot enforce coherency of no-snoop transactions. Device drivers
 must be careful about assuming that no-snoop transactions won't end
 up cached; see commit e02f5c1bb228 ("drm: disable uncached DMA
 optimization for ARM and arm64"). On x86 platforms, the host may or
 may not enforce coherency of no-snoop transactions with the physical
 IOMMU. But according to the above commit, on x86 a driver which
 assumes that no-snoop DMA is compatible with uncached CPU mappings
 will also work if the host enforces coherency.

 Although these issues are not specific to virtio-iommu, it could be
 used to facilitate discovery and configuration of no-snoop. This
 would require a new feature bit, PROBE property and ATTACH/MAP
 flags.

Interpreted in the *correct* context, I do think this is objectively less wrong than before. We can't guarantee that the underlying implementation will respect cacheable mappings, but it is true that we can do everything in our power to ask for them.

Reviewed-by: Robin Murphy robin.murphy@arm.com

Cc: stable@vger.kernel.org Fixes: e8ae0e140c05 ("vfio: Require that devices support DMA cache coherence") Signed-off-by: Jean-Philippe Brucker jean-philippe@linaro.org

---

Since v2 [1], I tried to refine the commit message. This fix is needed for v5.19 and v6.0.

I can improve the check once Robin's change [2] is merged: capable(IOMMU_CAP_CACHE_COHERENCY) could return dev->dma_coherent for case (1) above.

[1] https://lore.kernel.org/linux-iommu/20220818163801.1011548-1-jean-philippe@l... [2] https://lore.kernel.org/linux-iommu/d8bd8777d06929ad8f49df7fc80e1b9af32a41b5...

---

drivers/iommu/virtio-iommu.c | 11 +++++++++++ 1 file changed, 11 insertions(+)

diff --git a/drivers/iommu/virtio-iommu.c b/drivers/iommu/virtio-iommu.c index 08eeafc9529f..80151176ba12 100644 --- a/drivers/iommu/virtio-iommu.c +++ b/drivers/iommu/virtio-iommu.c @@ -1006,7 +1006,18 @@ static int viommu_of_xlate(struct device *dev, struct of_phandle_args *args) return iommu_fwspec_add_ids(dev, args->args, 1); } +static bool viommu_capable(enum iommu_cap cap) +{

• switch (cap) {
• case IOMMU_CAP_CACHE_COHERENCY:

• return true;
  

• default:

• return false;
  

• }

+}

• static struct iommu_ops viommu_ops = {
• .capable = viommu_capable, .domain_alloc = viommu_domain_alloc, .probe_device = viommu_probe_device, .probe_finalize = viommu_probe_finalize,