Re: [PATCH] PCI/P2PDMA: Add Intel Haswell client host bridge to the whitelist

[sashiko-bot@kernel.org]

> P2P DMA between two devices that do not share an upstream bridge is only
> permitted when the traffic goes through a host bridge that is listed in
> pci_p2pdma_whitelist[] with REQ_SAME_HOST_BRIDGE.
> 
> The Haswell client host bridge (8086:0c08, e.g. H97/H87/B85) is not in that
> list, and cpu_supports_p2pdma() returns false on Intel, so
> calc_map_type_and_dist() refuses P2P for any pair of devices behind
> different root ports of the same host bridge, even though that root complex
> handles the traffic correctly.
> 
> Measured on an ASUS H97-PRO with a Xeon E3-1231 v3 (Haswell, 8086:0c08 at
> 00:00.0) and two GPUs on separate root ports of that host bridge
> (00:01.0 and 00:1c.4):
> 
>   - pci_p2pdma_distance() < 0 before the change; peer access granted after
> [ ... ]
> pci_p2pdma_distance() all still apply. Tested on one board only, so the
> usual caveat applies - broad testing on other Haswell boards would be
> needed before this can be considered generally safe.
> 
> Signed-off-by: Scott Lee <dsix123@gmail.com>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20261005132651.18368-1-dsix123@gmail.com?part=1