From: Ankit Agrawal <ankita@xxxxxxxxxx> 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@xxxxxxxxxx/ [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@xxxxxxxxxx/ [4] Cc: Alex Williamson <alex.williamson@xxxxxxxxxx> Cc: Kevin Tian <kevin.tian@xxxxxxxxx> Cc: Jason Gunthorpe <jgg@xxxxxxxxxx> Cc: Vikram Sethi <vsethi@xxxxxxxxxx> Cc: Zhi Wang <zhiw@xxxxxxxxxx> Signed-off-by: Ankit Agrawal <ankita@xxxxxxxxxx> --- drivers/vfio/pci/nvgrace-gpu/main.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/drivers/vfio/pci/nvgrace-gpu/main.c b/drivers/vfio/pci/nvgrace-gpu/main.c index 25814006352d..a7fd018aa548 100644 --- a/drivers/vfio/pci/nvgrace-gpu/main.c +++ b/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 -- 2.34.1
