vfio/nvgrace-gpu: Convey kvm to map device memory region as noncached

The NVIDIA Grace Hopper GPUs have device memory that is supposed to be
used as a regular RAM. It is accessible through CPU-GPU chip-to-chip
cache coherent interconnect and is present in the system physical
address space. The device memory is split into two regions - termed
as usemem and resmem - in the system physical address space,
with each region mapped and exposed to the VM as a separate fake
device BAR [1].

Owing to a hardware defect for Multi-Instance GPU (MIG) feature [2],
there is a requirement - as a workaround - for the resmem BAR to
display uncached memory characteristics. Based on [3], on system with
FWB enabled such as Grace Hopper, the requisite properties
(uncached, unaligned access) can be achieved through a VM mapping (S1)
of NORMAL_NC and host mapping (S2) of MT_S2_FWB_NORMAL_NC.

KVM currently maps the MMIO region in S2 as MT_S2_FWB_DEVICE_nGnRE by
default. The fake device BARs thus displays DEVICE_nGnRE behavior in the
VM.

The following table summarizes the behavior for the various S1 and S2
mapping combinations for systems with FWB enabled [3].
S1           |  S2           | Result
NORMAL_NC    |  NORMAL_NC    | NORMAL_NC
NORMAL_NC    |  DEVICE_nGnRE | DEVICE_nGnRE

Recently a change was added that modifies this default behavior and
make KVM map MMIO as MT_S2_FWB_NORMAL_NC when a VMA flag
VM_ALLOW_ANY_UNCACHED is set [4]. Setting S2 as MT_S2_FWB_NORMAL_NC
provides the desired behavior (uncached, unaligned access) for resmem.

To use VM_ALLOW_ANY_UNCACHED flag, the platform must guarantee that
no action taken on the MMIO mapping can trigger an uncontained
failure. The Grace Hopper satisfies this requirement. So set
the VM_ALLOW_ANY_UNCACHED flag in the VMA.

Applied over next-20240227.
base-commit: 22ba90670a51

Link: https://lore.kernel.org/all/20240220115055.23546-4-ankita@nvidia.com/ [1]
Link: https://www.nvidia.com/en-in/technologies/multi-instance-gpu/ [2]
Link: https://developer.arm.com/documentation/ddi0487/latest/ section D8.5.5 [3]
Link: https://lore.kernel.org/all/20240224150546.368-1-ankita@nvidia.com/ [4]

Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Vikram Sethi <vsethi@nvidia.com>
Cc: Zhi Wang <zhiw@nvidia.com>
Signed-off-by: Ankit Agrawal <ankita@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20240229193934.2417-1-ankita@nvidia.comSigned-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/pci/nvgrace-gpu/main.c

@@ -160,8 +160,17 @@ static int nvgrace_gpu_mmap(struct vfio_device *core_vdev,
 	 * The carved out region of the device memory needs the NORMAL_NC
 	 * property. Communicate as such to the hypervisor.
 	 */
-	if (index == RESMEM_REGION_INDEX)
+	if (index == RESMEM_REGION_INDEX) {
+		/*
+		 * The nvgrace-gpu module has no issues with uncontained
+		 * failures on NORMAL_NC accesses. VM_ALLOW_ANY_UNCACHED is
+		 * set to communicate to the KVM to S2 map as NORMAL_NC.
+		 * This opens up guest usage of NORMAL_NC for this mapping.
+		 */
+		vm_flags_set(vma, VM_ALLOW_ANY_UNCACHED);
+
 		vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
+	}
 
 	/*
 	 * Perform a PFN map to the memory and back the device BAR by the