Re: [PATCH] virtio_pci_modern: fall back to 64 bits features for devices without an extended features space

[sashiko-bot@kernel.org]

> From: Peng Hao <flyingingpeng@tencent.com>
> 
> Since commit 69b9461512246 ("virtio_pci_modern: allow configuring
> extended features") the modern virtio-pci driver unconditionally
> accesses the whole 128 bits features space, i.e. it drives
> device_feature_select / guest_feature_select with the values 0..3.
> 
> Devices predating the extended features space only implement the
> legacy 64 bits one, and what they report for the selectors above it is
> not a valid features space.  Negotiating it makes the driver and the
> device end up with different features sets: on a smart NIC exposing a
> virtio_net device the link comes up but carries no traffic, while the
> same device works with a kernel that only accesses the low 64 bits.
> 
> Reading the features space has no side effect, so keep reading all of
> [ ... ]
> Conforming devices are unaffected: their extended words are neither
> all-ones nor an alias of the low ones, so the detection does not
> trigger.
> 
> Signed-off-by: Peng Hao <flyingingpeng@tencent.com>

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

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Sashiko AI review · https://sashiko.dev/#/patchset/20260911081133.16434-1-flyingpeng@tencent.com?part=1