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Subject: [PATCH] virtio: Improve queue_reset polarity to match to default reset state
Currently when driver initiates a queue reset, device is expected to communicate reset status to the driver by changing the value of the queue_reset register twice. First to return value other than 1 when reset is ongoing, later to return 1 when queue reset is completed. However initially during the device reset time the queue reset value is zero. queue_reset changes the value of the register to a different value on reset completion. Yet another time queue_reset value is expected to change when queue_select is reprogrammed. For example in below flow, a created virtqueue, which is disabled by driver leaves the queue state as queue_enable = 0, queue_reset = 1. example flow: a) 0,0 -> device init time value b) 1,0 -> vq is enabled by driver and working c) 1,1 -> vq is enabled, driver initiated reset d) 1,0 -> vq is enabled, but device is busy doing the reset e) 0,1 -> vq reset is completed in the device and VQ is now disabled (conflict with initial queue reset state #a above) On next iteration, when queue_select selects the same VQ again, without enablement, device is confused to return 1 or 0 because it was reset once before via queue_reset register. This demands complex device implementation to understand what should be returned for a VQ that is reset using queue_reset register vs other means. Instead, it is better and efficient to maintain the same VQ state on the device when queue reset is completed. new proposed flow: q_enable, q_reset A) 0, 0 -> default, device init time B) 1, 0 -> driver has enabled vq C) 1, 1 -> driver started q reset D) 1, 1 -> q_reset stays 1 until device is busy resetting vq (device communicates that its working on resetting VQ, consistent with #C) E) 0, 0 -> q_reset by device is completed, q got disabled (consistent with device init time #A) Otherwise a device needs to maintain complex state as, Current device side implementation requires something like, device.vq_reset_register_read() { if (vq.reset_started) { if (vq.reset_completed) return queue_reset = 1; else if (vq.reset_in_progress) return queue_reset = 0; } else { when vq.queue_select, and vq.queue_enable = 1, and if vq.reset_completed. { do extra queue_reset = 0; vq.reset_completed = 0; /* so that it can go back to original state */ } return queue_reset = 0; } } Compare to above, it is more efficient for device to do: device.vq_reset_register_read() { /* return current state of queue_reset; * if reset is ongoing, return 1. * if reset is completed or not done ever, return 0. */ return vq.queue_reset; } Hence, this patch proposes a simple change to have reset register polarity to be same as that of initial reset value. Fixes: https://github.com/oasis-tcs/virtio-spec/issues/139 Fixes: 12998e738621 ("virtio: pci support virtqueue reset") Signed-off-by: Parav Pandit <parav@nvidia.com> --- content.tex | 19 +++++++++++++------ 1 file changed, 13 insertions(+), 6 deletions(-) diff --git a/content.tex b/content.tex index 060bdab..77c1c66 100644 --- a/content.tex +++ b/content.tex @@ -1069,14 +1069,18 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport The driver MUST NOT write a 0 to \field{queue_enable}. If VIRTIO_F_RING_RESET has been negotiated, after the driver writes 1 to -\field{queue_reset} to reset the queue, it MUST verify that the queue -has been reset by reading back \field{queue_reset} and ensuring that it -is 1. The driver MAY re-enable the queue by writing a 1 to +\field{queue_reset} to reset the queue, driver MUST verify that the queue +has been reset by reading back \field{queue_reset} until device returns a value of 0. +Device continue to report value of 1 for the \field{queue_reset} until device finishes +the queue reset request. When the device completes the queue reset, \field{queue_reset} +and \field{queue_enable} set to zero, indicating +that specified queue is ready to be enabled again. The driver MAY re-enable the queue by writing a 1 to \field{queue_enable} after ensuring that the other virtqueue fields have been set up correctly. The driver MAY set driver-writeable queue configuration values to different values than those that were used before the queue reset. (see \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Virtqueue Reset}). + \subsubsection{Notification structure layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Notification capability} The notification location is found using the VIRTIO_PCI_CAP_NOTIFY_CFG @@ -2063,9 +2067,12 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi it finishes handling an interrupt and MUST NOT set any of the undefined bits in the value. If VIRTIO_F_RING_RESET has been negotiated, after the driver writes 1 to -\field{QueueReset} to reset the queue, it MUST verify that the queue -has been reset by reading back \field{QueueReset} and ensuring that it -is 1. The driver MAY re-enable the queue by writing a 1 to +\field{QueueReset} to reset the queue, driver MUST verify that the queue +has been reset by reading back \field{QueueReset} until device returns a value of 0. +Device continue to report value of 1 for the \field{QueueReset} until device finishes +the reset. When the device completes the queue reset, \field{QueueReset} and +\field{QueueReady} set to zero, indicating that specified queue is ready to be +enabled again. The driver MAY re-enable the queue by writing a 1 to \field{QueueReady} after ensuring that the other virtqueue fields have been set up correctly. The driver MAY set driver-writeable queue configuration values to different values than those that were used before the queue reset. -- 1.8.3.1
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