netdev - Re: [PATCH bpf-next] bpf: doc: update answer for 32-bit subregister question

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Date:   Thu, 30 May 2019 11:16:04 -0700
From:   Song Liu <liu.song.a23@...il.com>
To:     Jiong Wang <jiong.wang@...ronome.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        oss-drivers@...ronome.com
Subject: Re: [PATCH bpf-next] bpf: doc: update answer for 32-bit subregister question

On Thu, May 30, 2019 at 12:46 AM Jiong Wang <jiong.wang@...ronome.com> wrote:
>
> There has been quite a few progress around the two steps mentioned in the
> answer to the following question:
>
>   Q: BPF 32-bit subregister requirements
>
> This patch updates the answer to reflect what has been done.
>
> v1:
>  - Integrated rephrase from Quentin and Jakub.
>
> Reviewed-by: Quentin Monnet <quentin.monnet@...ronome.com>
> Reviewed-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> Signed-off-by: Jiong Wang <jiong.wang@...ronome.com>
> ---
>  Documentation/bpf/bpf_design_QA.rst | 30 +++++++++++++++++++++++++-----
>  1 file changed, 25 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/bpf/bpf_design_QA.rst b/Documentation/bpf/bpf_design_QA.rst
> index cb402c5..5092a2a 100644
> --- a/Documentation/bpf/bpf_design_QA.rst
> +++ b/Documentation/bpf/bpf_design_QA.rst
> @@ -172,11 +172,31 @@ registers which makes BPF inefficient virtual machine for 32-bit
>  CPU architectures and 32-bit HW accelerators. Can true 32-bit registers
>  be added to BPF in the future?
>
> -A: NO. The first thing to improve performance on 32-bit archs is to teach
> -LLVM to generate code that uses 32-bit subregisters. Then second step
> -is to teach verifier to mark operations where zero-ing upper bits
> -is unnecessary. Then JITs can take advantage of those markings and
> -drastically reduce size of generated code and improve performance.
> +A: NO

Add period "."?

> +
> +But some optimizations on zero-ing the upper 32 bits for BPF registers are
> +available, and can be leveraged to improve the performance of JIT compilers
> +for 32-bit architectures.

I guess it should be "improve the performance of JITed BPF programs for 32-bit
architectures"?

Thanks,
Song

> +
> +Starting with version 7, LLVM is able to generate instructions that operate
> +on 32-bit subregisters, provided the option -mattr=+alu32 is passed for
> +compiling a program. Furthermore, the verifier can now mark the
> +instructions for which zero-ing the upper bits of the destination register
> +is required, and insert an explicit zero-extension (zext) instruction
> +(a mov32 variant). This means that for architectures without zext hardware
> +support, the JIT back-ends do not need to clear the upper bits for
> +subregisters written by alu32 instructions or narrow loads. Instead, the
> +back-ends simply need to support code generation for that mov32 variant,
> +and to overwrite bpf_jit_needs_zext() to make it return "true" (in order to
> +enable zext insertion in the verifier).
> +
> +Note that it is possible for a JIT back-end to have partial hardware
> +support for zext. In that case, if verifier zext insertion is enabled,
> +it could lead to the insertion of unnecessary zext instructions. Such
> +instructions could be removed by creating a simple peephole inside the JIT
> +back-end: if one instruction has hardware support for zext and if the next
> +instruction is an explicit zext, then the latter can be skipped when doing
> +the code generation.
>
>  Q: Does BPF have a stable ABI?
>  ------------------------------
> --
> 2.7.4
>