vfio: Move the Intel no-snoop control off of IOMMU_CACHE

IOMMU_CACHE means "normal DMA to this iommu_domain's IOVA should be cache
coherent" and is used by the DMA API. The definition allows for special
non-coherent DMA to exist - ie processing of the no-snoop flag in PCIe
TLPs - so long as this behavior is opt-in by the device driver.

The flag is mainly used by the DMA API to synchronize the IOMMU setting
with the expected cache behavior of the DMA master. eg based on
dev_is_dma_coherent() in some case.

For Intel IOMMU IOMMU_CACHE was redefined to mean 'force all DMA to be
cache coherent' which has the practical effect of causing the IOMMU to
ignore the no-snoop bit in a PCIe TLP.

x86 platforms are always IOMMU_CACHE, so Intel should ignore this flag.

Instead use the new domain op enforce_cache_coherency() which causes every
IOPTE created in the domain to have the no-snoop blocking behavior.

Reconfigure VFIO to always use IOMMU_CACHE and call
enforce_cache_coherency() to operate the special Intel behavior.

Remove the IOMMU_CACHE test from Intel IOMMU.

Ultimately VFIO plumbs the result of enforce_cache_coherency() back into
the x86 platform code through kvm_arch_register_noncoherent_dma() which
controls if the WBINVD instruction is available in the guest.  No other
archs implement kvm_arch_register_noncoherent_dma() nor are there any
other known consumers of VFIO_DMA_CC_IOMMU that might be affected by the
user visible result change on non-x86 archs.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/2-v3-2cf356649677+a32-intel_no_snoop_jgg@nvidia.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

include/linux/intel-iommu.h

@@ -539,7 +539,6 @@ struct dmar_domain {
 
 	u8 has_iotlb_device: 1;
 	u8 iommu_coherency: 1;		/* indicate coherency of iommu access */
-	u8 iommu_snooping: 1;		/* indicate snooping control feature */
 	u8 force_snooping : 1;		/* Create IOPTEs with snoop control */
 
 	struct list_head devices;	/* all devices' list */