> Since commit 69b9461512246 ("virtio_pci_modern: allow configuring
> extended features"), vp_modern_set_extended_features() writes all 128
> feature bits on every device. Each 32-bit word requires one write to
> guest_feature_select and another to guest_feature, even when the upper
> 64 feature bits are all zero.
>
> Feature negotiation starts after a device reset, which clears the
> device-side driver features. There is therefore no need to write
> trailing zero 64-bit words on the first feature finalization.
>
> finalize_features() can, however, be called again without an intervening
> reset when a driver's validate callback changes the negotiated features.
> Remember how many words were written since the last reset and include
> that range in the next write, so that a repeated finalization can clear
> features which were previously enabled.
>
> This avoids four MMIO writes for devices which negotiate no feature
> above bit 63, while preserving the full-overwrite behavior when the
> negotiated feature range shrinks.
>
> 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
