Merge branch kvm-arm64/vfio-normal-nc into kvmarm/next

* kvm-arm64/vfio-normal-nc:
  : Normal-NC support for vfio-pci @ stage-2, courtesy of Ankit Agrawal
  :
  : KVM's policy to date has been that any and all MMIO mapping at stage-2
  : is treated as Device-nGnRE. This is primarily done due to concerns of
  : the guest triggering uncontainable failures in the system if they manage
  : to tickle the device / memory system the wrong way, though this is
  : unnecessarily restrictive for devices that can be reasoned as 'safe'.
  :
  : Unsurprisingly, the Device-* mapping can really hurt the performance of
  : assigned devices that can handle Gathering, and can be an outright
  : correctness issue if the guest driver does unaligned accesses.
  :
  : Rather than opening the floodgates to the full ecosystem of devices that
  : can be exposed to VMs, take the conservative approach and allow PCI
  : devices to be mapped as Normal-NC since it has been determined to be
  : 'safe'.
  vfio: Convey kvm that the vfio-pci device is wc safe
  KVM: arm64: Set io memory s2 pte as normalnc for vfio pci device
  mm: Introduce new flag to indicate wc safe
  KVM: arm64: Introduce new flag for non-cacheable IO memory

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

1 files changed, 18 insertions, 1 deletions

diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
index 1cbc990d42e0..df6f99bdf70d 100644
--- a/drivers/vfio/pci/vfio_pci_core.c
+++ b/drivers/vfio/pci/vfio_pci_core.c
@@ -1862,8 +1862,25 @@ int vfio_pci_core_mmap(struct vfio_device *core_vdev, struct vm_area_struct *vma
 	/*
 	 * See remap_pfn_range(), called from vfio_pci_fault() but we can't
 	 * change vm_flags within the fault handler.  Set them now.
+	 *
+	 * VM_ALLOW_ANY_UNCACHED: The VMA flag is implemented for ARM64,
+	 * allowing KVM stage 2 device mapping attributes to use Normal-NC
+	 * rather than DEVICE_nGnRE, which allows guest mappings
+	 * supporting write-combining attributes (WC). ARM does not
+	 * architecturally guarantee this is safe, and indeed some MMIO
+	 * regions like the GICv2 VCPU interface can trigger uncontained
+	 * faults if Normal-NC is used.
+	 *
+	 * To safely use VFIO in KVM the platform must guarantee full
+	 * safety in the guest where no action taken against a MMIO
+	 * mapping can trigger an uncontained failure. The assumption is
+	 * that most VFIO PCI platforms support this for both mapping types,
+	 * at least in common flows, based on some expectations of how
+	 * PCI IP is integrated. Hence VM_ALLOW_ANY_UNCACHED is set in
+	 * the VMA flags.
 	 */
-	vm_flags_set(vma, VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP);
+	vm_flags_set(vma, VM_ALLOW_ANY_UNCACHED | VM_IO | VM_PFNMAP |
+			VM_DONTEXPAND | VM_DONTDUMP);
 	vma->vm_ops = &vfio_pci_mmap_ops;
 
 	return 0;