Re: [PATCH 15/15] ebpf-docs: Add note about invalid instruction

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: dthaler1968@googlemail.com
Cc: bpf@vger.kernel.org, Dave Thaler <dthaler@microsoft.com>
Subject: Re: [PATCH 15/15] ebpf-docs: Add note about invalid instruction
Date: Fri, 30 Sep 2022 15:21:02 -0700	[thread overview]
Message-ID: <20220930222102.ulfwinlp7l4srphc@macbook-pro-4.dhcp.thefacebook.com> (raw)
In-Reply-To: <20220927185958.14995-15-dthaler1968@googlemail.com>

On Tue, Sep 27, 2022 at 06:59:58PM +0000, dthaler1968@googlemail.com wrote:
> From: Dave Thaler <dthaler@microsoft.com>
> 
> Signed-off-by: Dave Thaler <dthaler@microsoft.com>
> ---
>  Documentation/bpf/clang-notes.rst     | 5 +++++
>  Documentation/bpf/instruction-set.rst | 3 +++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/Documentation/bpf/clang-notes.rst b/Documentation/bpf/clang-notes.rst
> index 528feddf2..3c934421b 100644
> --- a/Documentation/bpf/clang-notes.rst
> +++ b/Documentation/bpf/clang-notes.rst
> @@ -20,6 +20,11 @@ Arithmetic instructions
>  For CPU versions prior to 3, Clang v7.0 and later can enable ``BPF_ALU`` support with
>  ``-Xclang -target-feature -Xclang +alu32``.  In CPU version 3, support is automatically included.
>  
> +Invalid instructions
> +====================
> +
> +Clang will generate the invalid ``BPF_CALL | BPF_X | BPF_JMP`` (0x8d) instruction if ``-O0`` is used.

I wouldn't call it invalid and it's not related to -O0.
It's a "reserved" instruction.
When we support indirect jumps that's what it would be.

> +
>  Atomic operations
>  =================
>  
> diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
> index 2ac8f0dae..af9dc0cc6 100644
> --- a/Documentation/bpf/instruction-set.rst
> +++ b/Documentation/bpf/instruction-set.rst
> @@ -303,6 +303,9 @@ with the remaining registers being ignored.  The definition of a helper function
>  is responsible for specifying the type (e.g., integer, pointer, etc.) of the value returned,
>  the number of arguments, and the type of each argument.
>  
> +Note that ``BPF_CALL | BPF_X | BPF_JMP`` (0x8d), where the helper function integer
> +would be read from a specified register, is not currently permitted.

I would say:
Note that ``BPF_CALL | BPF_X | BPF_JMP`` instruction is reserved and currently not permitted.

> +
>  Runtime functions
>  ~~~~~~~~~~~~~~~~~
>  Runtime functions are like helper functions except that they are not specific
> -- 
> 2.33.4
> 