[RFC] Improve the efficiency of needs_notification for queues
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This commit changes the way state is stored and logic is computed to determine whether the device should notify the guest driver about adding a descriptor chain to the used ring when the EVENT_IDX feature is negotiated. The `signalled_used` member of `QueueState`, currently an `Option<Wrapping<u16>>`, is replaced with an `Wrapping<u16>`, and the `needs_notification` implementation is altered as well. Signed-off-by: Alexandru Agache <[email protected]>
rbradford
approved these changes
Apr 21, 2022
andreeaflorescu
approved these changes
Apr 21, 2022
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This PR proposes an improvement to the efficiency of storing state and executing functionality related to the
needs_notificationqueue logic. It replaces the currentsignalled_used: Option<Wrapping<u16>>member of the state structure with anum_added: Wrapping<u16>(which keeps track of how many descriptor chains have been added to the used ring since the last call toneeds_notification), and also simplifies the logic inneeds_notification.A similar approach is used in the Linux virtio code as well (one example can be found here). It appears to bring noticeable performance improvements in use cases where
needs_notificationcalls constitute an increased proportion of the overall budget allocated for device model functionality (i.e. when using io_uring-based backends).