virtio — archive

[Michael S. Tsirkin]

Sometimes the device needs to only write out a single used descriptor after processing a batch of multiple available descriptors. This can happen when using descriptor chaining or with in-order use of descriptors. In this case, the device writes out a single used descriptor with the buffer id of the last descriptor in the batch. When this happens, used descriptors stop being contiguous in memory. As a result each used descriptor needs a separate memory write. For devices on the pci express bus, this incurs overhead of a separate write trasaction (e.g. pci express headers etc). This patch proposes a new feature: VIRTIO_F_RING_PACKED_USED. The idea is that both device and driver count all of the descriptors in a batch as if all of them are being written out when used. The resulting batch counter controls wrap around to beginning of the ring, which then serves as a synchronization point. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> --- content.tex 3 +++ packed-ring.tex 27 +++++++++++++++++++++++---- 2 files changed, 26 insertions(+), 4 deletions(-) diff --git a/content.tex b/content.tex index 47a4c6a..5b3121e 100644 --- a/content.tex +++ b/content.tex @@ -6247,6 +6247,9 @@ chapter{Reserved Feature Bits}label{sec:Reserved Feature Bits} item[VIRTIO_F_ORDER_PLATFORM(36)] This feature indicates that memory accesses by the driver and the device are ordered in a way described by the platform. + item[VIRTIO_F_RING_PACKED_USED(37)] This feature indicates + support for packed write out of used descriptors. Requires + VIRTIO_F_RING_PACKED. If this feature bit is negotiated, the ordering in effect for any memory accesses by the driver that need to be ordered in a specific way diff --git a/packed-ring.tex b/packed-ring.tex index ea92543..f8a2676 100644 --- a/packed-ring.tex +++ b/packed-ring.tex @@ -131,17 +131,36 @@ subsection{Polling of available and used descriptors} poll (or test) a single location in memory: the next device descriptor after the one they processed previously, in circular order. +subsection{Skipping Used Descriptors} +label{sec:Packed Virtqueues / Skipping Used Descriptors} Sometimes the device needs to only write out a single used descriptor after processing a batch of multiple available descriptors. As described in more detail below, this can happen when using descriptor chaining or with in-order -use of descriptors. In this case, the device writes out a used -descriptor with the buffer id of the last descriptor in the group. -After processing the used descriptor, both device and driver then +use of descriptors. In this case, the device writes out a single used +descriptor with the buffer id of the last descriptor in the batch. + +If VIRTIO_F_RING_PACKED_USED has not been negotiated, +after processing the used descriptor, both device and driver then skip forward in the ring the number of the remaining descriptors -in the group until processing (reading for the driver and writing +in the batch until processing (reading for the driver and writing for the device) the next used descriptor. +If VIRTIO_F_RING_PACKED_USED has been negotiated, +both the device and the driver are expected to maintain, +internally, a 16-bit used batch counter initialized to 0. +Each time a batch of descriptors is used, the batch counter +is incremented by the number of descriptors in the batch. +If after processing the used descriptor, the batch counter +does not exceed the ring size, the next used descriptor +is written immediately adjacent to the current one, in ring +order. +When after processing the used descriptor, the batch counter +exceeds the ring size, the batch counter is decremented by the +ring size; both device and driver then skip to the +beginning (offset 0) of the ring until processing (reading for +the driver and writing for the device) the next used descriptor. + subsection{Write Flag} label{sec:Packed Virtqueues / Write Flag} -- MST