Re: [PATCH bpf-next] bpftool: Probe for ISA v4 instruction set extension

[Quentin Monnet <qmo@kernel.org>]

On 09/12/2024 10:26, Simone Magnani wrote:
> This patch introduces a new probe to check whether the kernel supports
> instruction set extensions v4. The v4 extension comprises several new
> instructions: BPF_{SDIV,SMOD} (signed div and mod), BPF_{LD,LDX,ST,STX,MOV}
> (sign-extended load/store/move), 32-bit BPF_JA (unconditional jump),
> target-independent BPF_ALU64 BSWAP (byte-swapping 16/32/64). These have
> been introduced in the following commits respectively:
> 
> * ec0e2da ("bpf: Support new signed div/mod instructions.")
> * 1f9a1ea ("bpf: Support new sign-extension load insns")
> * 8100928 ("bpf: Support new sign-extension mov insns")
> * 4cd58e9 ("bpf: Support new 32bit offset jmp instruction")
> * 0845c3d ("bpf: Support new unconditional bswap instruction")
> 
> Support in bpftool for previous ISA extensions were added in commit
> 0fd800b2 ("bpftool: Probe for instruction set extensions"). These probes
> are useful for userspace BPF projects that want to use newer
> instruction set extensions on newer kernels, to reduce the programs'
> sizes or their complexity. LLVM provides the mcpu=v4 option since commit
> "[BPF] support for BPF_ST instruction in codegen"
> (https://github.com/llvm/llvm-project/commit/8f28e8069c4ba1110daee8bddc4d5049b6d4646e).
> 
> Signed-off-by: Simone Magnani <simone.magnani@isovalent.com>
> ---
>  tools/bpf/bpftool/feature.c  | 23 +++++++++++++++++++++++
>  tools/include/linux/filter.h | 10 ++++++++++
>  2 files changed, 33 insertions(+)
> 
> diff --git a/tools/bpf/bpftool/feature.c b/tools/bpf/bpftool/feature.c
> index 4dbc4fcdf473..24fecdf8e430 100644
> --- a/tools/bpf/bpftool/feature.c
> +++ b/tools/bpf/bpftool/feature.c
> @@ -885,6 +885,28 @@ probe_v3_isa_extension(const char *define_prefix, __u32 ifindex)
>  			   "V3_ISA_EXTENSION");
>  }
> 
> +/*
> + * Probe for the v4 instruction set extension introduced in commit 1f9a1ea821ff
> + * ("bpf: Support new sign-extension load insns").
> + */
> +static void
> +probe_v4_isa_extension(const char *define_prefix, __u32 ifindex)
> +{
> +	struct bpf_insn insns[5] = {
> +		BPF_MOV64_IMM(BPF_REG_0, 0),
> +		BPF_JMP32_IMM(BPF_JEQ, BPF_REG_0, 1, 1),
> +		BPF_JMP32_A(1),
> +		BPF_MOV64_IMM(BPF_REG_0, 1),
> +		BPF_EXIT_INSN()
> +	};
> +
> +	probe_misc_feature(insns, ARRAY_SIZE(insns),
> +			   define_prefix, ifindex,
> +			   "have_v4_isa_extension",
> +			   "ISA extension v4",
> +			   "V4_ISA_EXTENSION");
> +}
> +
>  static void
>  section_system_config(enum probe_component target, const char *define_prefix)
>  {
> @@ -1029,6 +1051,7 @@ static void section_misc(const char *define_prefix, __u32 ifindex)
>  	probe_bounded_loops(define_prefix, ifindex);
>  	probe_v2_isa_extension(define_prefix, ifindex);
>  	probe_v3_isa_extension(define_prefix, ifindex);
> +	probe_v4_isa_extension(define_prefix, ifindex);
>  	print_end_section();
>  }
> 
> diff --git a/tools/include/linux/filter.h b/tools/include/linux/filter.h
> index 65aa8ce142e5..a2962fc56f27 100644
> --- a/tools/include/linux/filter.h
> +++ b/tools/include/linux/filter.h
> @@ -75,6 +75,16 @@
>  		.off   = 0,					\
>  		.imm   = LEN })
> 
> +/* Unconditional jumps, gotol pc + imm32 */
> +
> +#define BPF_JMP32_A(IMM)					\
> +	((struct bpf_insn) {					\
> +		.code  = BPF_JMP32 | BPF_JA,			\
> +		.dst_reg = 0,					\
> +		.src_reg = 0,					\
> +		.off   = 0,					\
> +		.imm   = IMM })
> +
>  /* Short form of mov, dst_reg = src_reg */
> 
>  #define BPF_MOV64_REG(DST, SRC)					\


Nit: Can you please move this macro after BPF_JMP_A() in filter.h, so
that it remains consistent with the order in include/linux/filter.h?

The bpftool change looks good, thanks!

Please Cc other BPF maintainers too for bpftool patches.

Quentin