This is version 1.0 of an informational document containing recommended conventions and guidelines for producing portable BPF program binaries.
BPF has 10 general purpose registers and a read-only frame pointer register, all of which are 64-bits wide.
The BPF calling convention is defined as:
R0: return value from function calls, and exit value for BPF programs
R1 - R5: arguments for function calls
R6 - R9: callee saved registers that function calls will preserve
R10: read-only frame pointer to access stack
R0 - R5 are scratch registers and BPF programs needs to spill/fill them if necessary across calls.