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docs: add documentation for GPUDirect P2P

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Signed-off-by: Thomas Barrett <tbarrett1200@icloud.com>
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Thomas Barrett 2024-04-27 04:52:26 +00:00 committed by Rob Bradford
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docs/vfio.md
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@ -126,6 +126,39 @@ VM, otherwise this could cause some functional and security issues.
### Advanced Configuration Options
When using NVIDIA GPUs in a VFIO passthrough configuration, advanced
configuration options are supported to enable GPUDirect P2P DMA over
PCIe. When enabled, loads and stores between GPUs use native PCIe
peer-to-peer transactions instead of a shared memory buffer. This drastically
decreases P2P latency between GPUs. This functionality is supported by
cloud-hypervisor on NVIDIA Turing, Ampere, Hopper, and Lovelace GPUs.
The NVIDIA driver does not enable GPUDirect P2P over PCIe within guests
by default because hardware support for routing P2P TLP between PCIe root
ports is optional. PCIe P2P should always be supported between devices
on the same PCIe switch. The `x_nv_gpudirect_clique` config argument may
be used to signal support for PCIe P2P traffic between NVIDIA VFIO endpoints.
The guest driver assumes that P2P traffic is supported between all endpoints
that are part of the same clique.
```
--device path=/sys/bus/pci/devices/0000:01:00.0/,x_nv_gpudirect_clique=0
```
The following command can be run on the guest to verify that GPUDirect P2P is
correctly enabled.
```
nvidia-smi topo -p2p r
GPU0 GPU1 GPU2 GPU3 GPU4 GPU5 GPU6 GPU7
GPU0 X OK OK OK OK OK OK OK
GPU1 OK X OK OK OK OK OK OK
GPU2 OK OK X OK OK OK OK OK
GPU3 OK OK OK X OK OK OK OK
GPU4 OK OK OK OK X OK OK OK
GPU5 OK OK OK OK OK X OK OK
GPU6 OK OK OK OK OK OK X OK
GPU7 OK OK OK OK OK OK OK X
```
Some VFIO devices have a 32-bit mmio BAR. When using many such devices, it is
possible to exhaust the 32-bit mmio space available on a PCI segment. The
following example demonstrates an example device with a 16 MiB 32-bit mmio BAR.