> 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 <[email protected]>
Sashiko has reviewed this patch and found no issues. It looks great!
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https://sashiko.dev/#/patchset/[email protected]?part=1
