+Packet data access (BPF_ABS, BPF_IND)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Two non-generic instructions: (BPF_ABS | <size> | BPF_LD) and
+(BPF_IND | <size> | BPF_LD) which are used to access packet data.
+Register R6 is an implicit input that must contain pointer to sk_buff.
+Register R0 is an implicit output which contains the data fetched
+from the packet. Registers R1-R5 are scratch registers and must not
+be used to store the data across BPF_ABS | BPF_LD or BPF_IND | BPF_LD
+instructions. These instructions have implicit program exit condition
+as well. When eBPF program is trying to access the data beyond
+the packet boundary, the interpreter will abort the execution of the program.
+
+BPF_IND | BPF_W | BPF_LD is equivalent to:
+ R0 = ntohl(\*(u32 \*) (((struct sk_buff \*) R6)->data + src_reg + imm32))
+
+eBPF maps
+^^^^^^^^^
+
+eBPF maps are provided for sharing data between kernel and user-space.
+Currently implemented types are hash and array, with potential extension to
+support bloom filters, radix trees, etc. A map is defined by its type,
+maximum number of elements, key size and value size in bytes. eBPF syscall
+supports create, update, find and delete functions on maps.
+
+Function calls
+^^^^^^^^^^^^^^
+
+Function call arguments are passed using up to five registers (R1 - R5).
+The return value is passed in a dedicated register (R0). Four additional
+registers (R6 - R9) are callee-saved, and the values in these registers
+are preserved within kernel functions. R0 - R5 are scratch registers within
+kernel functions, and eBPF programs must therefor store/restore values in
+these registers if needed across function calls. The stack can be accessed
+using the read-only frame pointer R10. eBPF registers map 1:1 to hardware
+registers on x86_64 and other 64-bit architectures. For example, x86_64
+in-kernel JIT maps them as
+
+::
+
+ R0 - rax
+ R1 - rdi
+ R2 - rsi
+ R3 - rdx
+ R4 - rcx
+ R5 - r8
+ R6 - rbx
+ R7 - r13
+ R8 - r14
+ R9 - r15
+ R10 - rbp
+
+since x86_64 ABI mandates rdi, rsi, rdx, rcx, r8, r9 for argument passing
+and rbx, r12 - r15 are callee saved.
+
+Program start
+^^^^^^^^^^^^^
+
+An eBPF program receives a single argument and contains
+a single eBPF main routine; the program does not contain eBPF functions.
+Function calls are limited to a predefined set of kernel functions. The size
+of a program is limited to 4K instructions: this ensures fast termination and
+a limited number of kernel function calls. Prior to running an eBPF program,
+a verifier performs static analysis to prevent loops in the code and
+to ensure valid register usage and operand types.