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docs: Document VFIO mmap BAR exclusion

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Explain how operators can exclude selected VFIO BARs from mmap
when they know their workloads do not require MMIO access to those
regions.

Documenting the option separately keeps the new device argument
discoverable and calls out the valid BAR index range.

Signed-off-by: Damian Barabonkov <dbctl@pm.me>
Assisted-by: OpenCode:gpt-5.5
This commit is contained in:
Damian Barabonkov
2026-05-06 08:17:45 +02:00
committed by Rob Bradford
parent 41e12e9d6a
commit a858a1f115
1 changed files with 11 additions and 0 deletions
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docs/vfio.md
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@@ -159,6 +159,17 @@ nvidia-smi topo -p2p r
GPU7 OK OK OK OK OK OK OK X
```
Some VFIO devices expose BARs that should not be mmapped by the VMM even when
the kernel reports them as mappable. The `x_exclude_mmap_bars` config argument can
be used to skip mmap for specific BAR indices.
BAR indices must be between 0 and 5 inclusive. The following example disables
mmap for BAR 2 of the assigned device:
```
--device path=/sys/bus/pci/devices/0000:01:00.0/,x_exclude_mmap_bars=[2]
```
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.