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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.com Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.