Re: [PATCH] pci: Support preferring MMCONFIG for all config space accesses

[sashiko-bot@kernel.org]

> Even when MMCONFIG is available, PCIe currently defaults to IO-based
> accesses for the first 256 bytes of config space.
> 
> IO-based accesses are serialized, and can cause substantial scalability
> issues when initializing a large number of devices on a system with many
> CPUs. (Those often coincide, because many systems have PCIe devices per
> CPU.)
> 
> Add a `pci=prefer_mmconf` option to use MMCONFIG for all config space
> accesses. Also add a Kconfig option (default n) to make that the
> default, allowing a boot-time opt out with `pci=no_prefer_mmconf`.
> 
> Tested on an AWS c8i.48xlarge system, with 192 CPUs, over five boots
> with and five boots without.
> 
> Without:
> PCI init 266ms
> Boot to minimal /init 726ms
> 
> With:
> PCI init 213ms (~20% faster)
> Boot to minimal /init 661ms (~9% faster)
> 
> Signed-off-by: Josh Triplett <josh@joshtriplett.org>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/a7d2b402784a409f5564c1fdce77a41e09f29280.1793087687.git.josh@joshtriplett.org?part=1