[PATCH v2] iommu/virtio: Fix interaction with VFIO

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

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

Non-coherent accesses are not currently a concern for virtio-iommu
because host OSes only assign coherent devices, and the guest does not
enable PCIe no-snoop. Nevertheless, we can summarize here the possible
support for 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. Some architecture do not
    support this either (whether no-snoop is supported by an Arm system
    is not discoverable by software). If Linux did enable no-snoop in
    endpoints on x86, then virtio-iommu would need additional feature,
    PROBE property, ATTACH and/or MAP flags to support enforcing snoop.

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

Since v1 [1], I added some details to the commit message. This fix is
still 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/20220714111059.708735-1-jean-philippe@linaro.org/
[2] https://lore.kernel.org/linux-iommu/d8bd8777d06929ad8f49df7fc80e1b9af32a41b5.1660574547.git.robin.murphy@arm.com/
---
 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,
-- 
2.37.1

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization