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Subject: [PATCH v1 1/1] vfio/nvgrace-gpu: Convey kvm that the device is wc safe
Date: Wed, 28 Feb 2024 19:48:01 +0000
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From: Ankit Agrawal <ankita@nvidia.com>

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_WB    |  NORMAL_NC    | NORMAL_NC
NORMAL_WT    |  NORMAL_NC    | NORMAL_NC
NORMAL_NC    |  NORMAL_NC    | NORMAL_NC
NORMAL_WB    |  DEVICE_nGnRE | DEVICE_nGnRE
NORMAL_WT    |  DEVICE_nGnRE | DEVICE_nGnRE
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. Setting S2 as MT_S2_FWB_NORMAL_NC
provides the desired behavior (uncached, unaligned access) for resmem.

Such setting is extended to the usemem as a middle-of-the-road
setting to take it closer to the desired final system memory
characteristics (cached, unaligned). This will eventually be
fixed with the ongoing proposal [4].

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/20230907181459.18145-2-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>
---
 drivers/vfio/pci/nvgrace-gpu/main.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/drivers/vfio/pci/nvgrace-gpu/main.c b/drivers/vfio/pci/nvgrace-gpu/main.c
index 25814006352d..5539c9057212 100644
--- a/drivers/vfio/pci/nvgrace-gpu/main.c
+++ b/drivers/vfio/pci/nvgrace-gpu/main.c
@@ -181,6 +181,24 @@ static int nvgrace_gpu_mmap(struct vfio_device *core_vdev,
 
 	vma->vm_pgoff = start_pfn;
 
+	/*
+	 * The VM_ALLOW_ANY_UNCACHED 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). This also
+	 * unlocks memory-like operations such as unaligned accesses.
+	 * This setting suits the fake BARs as they are expected to
+	 * demonstrate such properties within the guest.
+	 *
+	 * 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. The nvgrace-gpu
+	 * however is safe in that the platform guarantees that no
+	 * action taken on the MMIO mapping can trigger an uncontained
+	 * failure. Hence VM_ALLOW_ANY_UNCACHED is set in the VMA flags.
+	 */
+	vm_flags_set(vma, VM_ALLOW_ANY_UNCACHED);
+
 	return 0;
 }
 
-- 
2.34.1