Re: [PATCH v2 1/1] vfio/nvgrace-gpu: Convey kvm to map device memory region as noncached

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Feb 29, 2024 at 07:39:34PM +0000, ankita@nvidia.com wrote:
> 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_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>
> ---
>  drivers/vfio/pci/nvgrace-gpu/main.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason