On 21/09/17 17:40, Y Song wrote: > On Thu, Sep 21, 2017 at 9:24 AM, Edward Cree <ec...@solarflare.com> wrote: >> On 21/09/17 16:52, Alexei Starovoitov wrote: >>> imo >>> (u16) r4 endian be >>> isn't intuitive. >>> Can we come up with some better syntax? >>> Like >>> bswap16be r4 >>> bswap32le r4 >> Hmm, I don't like these, since bswapbe is a swap on *le* and a nop on be. >>> or >>> >>> to_be16 r4 >>> to_le32 r4 >> And the problem here is that it's not just to_be, it's also from_be. > Could you explain what is "from_be" here? Do not quite understand. Taking the example of a little-endian processor: cpu_to_be16() is a byte-swap, converting a u16 (cpu-endian) to a __be16. be16_to_cpu(), to convert a __be16 to a u16, is *also* a byte-swap. Meanwhile, cpu_to_le16() and le16_to_cpu() are both no-ops.
More generally, the conversions between cpu-endian and fixed-endian for any given size are self-inverses. eBPF takes advantage of this by only having a single opcode for both the "to" and "from" direction. So to specify an endianness conversion, you need only the size and the fixed endianness (le or be), not the to/from direction. Conversely, when disassembling one of these instructions, you don't know whether it's a cpu_to_be16() or a be16_to_cpu(), because they both look the same at an instruction level (they only differ in what types the programmer thought of the register as holding before and after).
