Re: [PATCH v2] virtio_pci_modern: skip trailing zero feature dwords

[sashiko-bot@kernel.org]

> Since commit 69b9461512246 ("virtio_pci_modern: allow configuring
> extended features"), vp_modern_set_extended_features() writes all four
> feature dwords on every device, even when the upper ones are zero.
> 
> Feature negotiation follows a device reset, which clears the device-side
> driver features, so trailing zero dwords need not be written at all.
> finalize_features() can be called again without an intervening reset,
> though, when a driver's validate callback narrows the features, so also
> write any dword written since the last reset, to clear what the previous
> call had enabled.
> 
> Devices negotiating nothing above bit 63 save four MMIO writes; those
> using the 64..95 range (e.g. the UDP tunnel GSO features) save two.
> Counting dwords rather than 64-bit words is what makes the latter work:
> with VIRTIO_F_VERSION_1 at bit 32 the second dword is set on every modern
> device, so a qword count never drops below two.
> 
> Signed-off-by: Peng Hao <flyingpeng@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/20260924071412.6765-1-flyingpeng@tencent.com?part=1