[iovisor-dev] [PATCH RFC 0/4] Initial 32-bit eBPF encoding support

Fulvio Risso fulvio.risso at polito.it
Tue Sep 19 04:49:09 UTC 2017

Dear Jiong,

that's a great work.
I havent' gone through the whole patches, but it seems to me that the 
documentation is not that much.
 From my past experiences, putting your hands into a compiler without at 
least some high-level documentation that presents how it works, it would 
be a nightmare.
Even something like what you wrote in this email is valuable; of course, 
also how to turn this feature on.



On 18/09/2017 13:47, Jiong Wang via iovisor-dev wrote:
> Hi,
>    Currently, LLVM eBPF backend always generate code in 64-bit mode, this may
> cause troubles when JITing to 32-bit targets.
>    For example, it is quite common for XDP eBPF program to access some packet
> fields through base + offset that the default eBPF will generate BPF_ALU64 for
> the address formation, later when JITing to 32-bit hardware, BPF_ALU64 needs
> to be expanded into 32 bit ALU sequences even though the address space is
> 32-bit that the high bits is not significant.
>    While a complete 32-bit mode implemention may need an new ABI (something like
> -target-abi=ilp32), this patch set first add some initial code so we could
> construct 32-bit eBPF tests through hand-written assembly.
>    A new 32-bit register set is introduced, its name is with "w" prefix and LLVM
> assembler will encode statements like "w1 += w2" into the following 8-bit code
> field:
>      BPF_ADD | BPF_X | BPF_ALU
> BPF_ALU will be used instead of BPF_ALU64.
>    NOTE, currently you can only use "w" register with ALU statements, not with
> others like branches etc as they don't have different encoding for 32-bit
> target.
>    Comments?
> *** BLURB HERE ***
> Jiong Wang (4):
>    Improve instruction encoding descriptions
>    Improve class inheritance in instruction patterns
>    New 32-bit register set
>    Initial 32-bit ALU (BPF_ALU) encoding support in assembler
>   lib/Target/BPF/BPFInstrFormats.td               |  84 +++-
>   lib/Target/BPF/BPFInstrInfo.td                  | 506 +++++++++++-------------
>   lib/Target/BPF/BPFRegisterInfo.td               |  74 +++-
>   lib/Target/BPF/Disassembler/BPFDisassembler.cpp |  15 +
>   test/MC/BPF/insn-unit-32.s                      |  53 +++
>   5 files changed, 427 insertions(+), 305 deletions(-)
>   create mode 100644 test/MC/BPF/insn-unit-32.s

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