On Thu, Feb 29, 2024 at 07:39:34PM +0000, ankita@xxxxxxxxxx wrote: > 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(-) Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx> Jason
