1. Receiver ‘packet-splitting’
A look at how a driver can cause the 82573L NIC to separate a packet’s headers from its data

2. NIC can do packet-parsing
Intel’s newest gigabit Ethernet controllers offer an enhancement to the ‘extended’ Receive Descriptor, called ‘packet-split’ format, which empowers the hardware to recognize the packet ‘headers’ used with the most common network protocols and to automatically separate those headers from their accompanying packet ‘data’

3. ‘Extended’ RX-Descriptors
CPU writes this, NIC reads it:
NIC writes this, CPU reads it:
Base-address (64-bits)
Packet-
checksum IP
identification
MRQ
(multiple receive queues)
reserved (=0)
VLAN
tag
Packet-
length
Extended
errors
Extended
status
The device-driver initializes
the ‘base-address’ field
with the physical address
of a packet-buffer, and
it initializes the ‘reserved’
field with a zero-value…
… the network hardware
will later modify both fields
The network controller
will ‘write-back’ the
values for these fields
when it has transferred
a received packet’s data
into the packet-buffer

4. ‘Packet-Split’ RX-Descriptors
CPU writes this, NIC reads it:
NIC writes this, CPU reads it:
Base-address 0 (64-bits)
Packet-
checksum IP
identification
MRQ
(multiple receive queues)
Base-address 1 (64-bits)
VLAN
tag
Packet-
Length 0
Extended
errors
Extended
status
Base-address 2 (64-bits)
Packet
Length 3
Packet
Length 2
Packet
Length 1 S P
Header
Length
Base-address 3 (64-bits)
reserved 1
The network controller will
‘write-back’ values to these
fields when it has transferred
a received packet’s data
into those packet-buffers
The device-driver initializes
four ‘base-address’ fields
(‘even-numbered’ addresses)

5. Same ‘Extended’ Status/Errors A C K U D P V I P V P I F I P C S T C P S U D P C S V P I X S M E O P D
RXE
IPE
TCPE
SEQ SE CE

6. Syntax modifications for ‘fetch’
typedef struct {
unsigned long long base_addr0;
unsigned long long base_addr1;
unsigned long long base_addr2;
unsigned long long base_addr3;
} RX_DESC_FETCH;

7. Syntax modifications for ‘store’
typedef struct {
unsigned int mrq;
unsigned short ip_identification;
unsigned short packet_chksum;
unsigned int desc_status:20;
unsigned int desc_errors:12;
unsigned short packet_length0;
unsigned short vlan_tag;
unsigned short header_length;
unsigned short packet_length1;
unsigned short packet_length2;
unsigned short packet_length3
unsigned long long reserved;
} RX_DESC_STORE;

8. Same syntax for the ‘union’
typedef union {
RX_DESC_FETCH rxf;
RX_DESC_STORE rxs;
} RX_DESCRIPTOR;

9. NIC Registers involved D T Y P
RCTL
Device Control register
Reserved (=0) E X T N
RFCTL
Receive Filter Control register
LEN3
(1KB)
LEN2
(1KB)
LEN1
(1KB)
LEN0
(128B)
PSRCTL
Packet-Split Receive Control register

10. Each descriptor has four buffers
Packet-Split Rx-descriptor
base_addr0
base_addr1
base_addr2
base_addr3
buffer3
buffer2
buffer1
buffer0
Four buffers are allocated for receiving one packet

11. Refresh for ‘reuse’
As with the ‘extended’ receive-descriptors, it is necessary for a device-driver to setup each ‘packet-split’ receive-descriptor any time it is going to be ‘reused’, since prior buffer-addresses get overwritten during a packet-reception by the network controller
So driver needs a formula for recalculating buffer-addresses, or use a ‘shadow’ array

12. Kernel-memory layout Sixteen Rx-descriptors (32-bytes each)
Sixty-four receive-buffers (1024-bytes each)
512
bytes
65536
bytes
KMEM_SIZE (= bytes)
kmem

13. Caveats Short packets are not always ‘split’
Unrecognized packet-headers may not be separated from accompanying packet-data
Demonstrating the packet-split capability will require us to devise a way to transmit packets which have the TCP/UDP and IP packet-headers that the NIC recognizes

14. TIMEOUT for an in-class demonstration
Our ‘pktsplit.c’ demo
We created a ‘minimal’ kernel-module for demonstrating the NIC’s ‘packet-splitting’ capabilities
TIMEOUT for an in-class demonstration

15. In-class exercise
Can you enhance our ‘pktsplit.c’ module so that its Receive-Descriptor Queue will function automatically as a ring-buffer (as happens in our ‘extended.c’ example)?
Your best option for this is to install an ISR which will reinitialize some Rx-Descriptors (and advance the RDT index) each time an RXDMT0 interrupt gets triggered