readv + MRG_RXBUF #4813
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readv + MRG_RXBUF #4813
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Add new `IovRingBuffer` ring buffer type that is tailored for holding `struct iovec` objects that point to guest memory for IO. The `struct iovec` objects represent the memory that the guest passed to us as `Descriptors` in a VirtIO queue for performing some I/O operation. We plan to use this type to describe the guest memory we have available for doing network RX. This should facilitate us in optimizing the reception of data from the TAP device using `readv`, thus avoiding a memory copy. Co-authored-by: Babis Chalios <[email protected]> Signed-off-by: Egor Lazarchuk <[email protected]>
Add generic fixed size ring buffer type. The main difference from a standard `VecDequeue` is the use of `u32` as indexes. This shrinks the size of the ring buffer from 32 bytes for `VecDequeue` to 24 bytes for `RingBuffer`. Signed-off-by: Egor Lazarchuk <[email protected]>
Split `add_used` internals into 2 new functions: `write_used_element` and `advance_used_ring`. This will be used in next commits to optimize RX path of net device. Signed-off-by: Egor Lazarchuk <[email protected]>
Right now, we are performing two copies for writing a frame from the TAP device into guest memory: - read the frame into an internal array held by the Net device - copy that array into a buffers of a DescriptorChain. In order to avoid this double copy we can use the readv system call to read directly from the TAP device into the buffers described by DescriptorChain. The main challenge with this is that DescriptorChain objects describe memory that is at least 65562 bytes long when guest TSO4, TSO6 or UFO are enabled or 1526 otherwise and parsing the chain includes overhead which we pay even if the frame we are receiving is much smaller than these sizes. PR firecracker-microvm#4748 reduced the overheads involved with parsing DescriptorChain objects. To further avoid this overhead, move the parsing of DescriptorChain objects out of the hot path of process_rx() where we are actually receiving a frame into process_rx_queue_event() where we get the notification that the guest added new buffers for network RX. Co-authored-by: Babis Chalios <[email protected]> Signed-off-by: Egor Lazarchuk <[email protected]>
Now virtio-net device VIRTIO_NET_F_MRG_RXBUF feature which allows it to write single packet into multiple descriptor chains. The amount of descriptor chains (also known as heads) is written into the `virtio_net_hdr_v1` structure which is located at the very begging of the packet. Signed-off-by: Egor Lazarchuk <[email protected]>
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Changes
Switch virtio-net device to use
readvto read from a tap device and implementMRG_RXBUFflag.Reason
Improve performance of the network device.
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