From: Paul Chaignon <paul@isovalent.com>
Date: Tue, 4 Jan 2022 19:00:13 +0100
Subject: bpftool: Probe for instruction set extensions
Patch-mainline: v5.17-rc1
Git-commit: 0fd800b2456cf90ed738a1260b53acaa8843b5ae
References: jsc#PED-1368

This patch introduces new probes to check whether the kernel supports
instruction set extensions v2 and v3. The first introduced eBPF
instructions BPF_J{LT,LE,SLT,SLE} in commit 92b31a9af73b ("bpf: add
BPF_J{LT,LE,SLT,SLE} instructions"). The second introduces 32-bit
variants of all jump instructions in commit 092ed0968bb6 ("bpf:
verifier support JMP32").

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 already provides an mcpu=probe option to
automatically probe the kernel and select the newest-supported
instruction set extension. That is however not flexible enough for all
use cases. For example, in Cilium, we only want to use the v3
instruction set extension on v5.10+, even though it is supported on all
kernels v5.1+.

Signed-off-by: Paul Chaignon <paul@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/3bfedcd9898c1f41ac67ca61f144fec84c6c3a92.1641314075.git.paul@isovalent.com
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
---
 tools/bpf/bpftool/feature.c |   44 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

--- a/tools/bpf/bpftool/feature.c
+++ b/tools/bpf/bpftool/feature.c
@@ -708,6 +708,48 @@ probe_bounded_loops(const char *define_p
 			   "BOUNDED_LOOPS");
 }
 
+/*
+ * Probe for the v2 instruction set extension introduced in commit 92b31a9af73b
+ * ("bpf: add BPF_J{LT,LE,SLT,SLE} instructions").
+ */
+static void
+probe_v2_isa_extension(const char *define_prefix, __u32 ifindex)
+{
+	struct bpf_insn insns[4] = {
+		BPF_MOV64_IMM(BPF_REG_0, 0),
+		BPF_JMP_IMM(BPF_JLT, BPF_REG_0, 0, 1),
+		BPF_MOV64_IMM(BPF_REG_0, 1),
+		BPF_EXIT_INSN()
+	};
+
+	probe_misc_feature(insns, ARRAY_SIZE(insns),
+			   define_prefix, ifindex,
+			   "have_v2_isa_extension",
+			   "ISA extension v2",
+			   "V2_ISA_EXTENSION");
+}
+
+/*
+ * Probe for the v3 instruction set extension introduced in commit 092ed0968bb6
+ * ("bpf: verifier support JMP32").
+ */
+static void
+probe_v3_isa_extension(const char *define_prefix, __u32 ifindex)
+{
+	struct bpf_insn insns[4] = {
+		BPF_MOV64_IMM(BPF_REG_0, 0),
+		BPF_JMP32_IMM(BPF_JLT, BPF_REG_0, 0, 1),
+		BPF_MOV64_IMM(BPF_REG_0, 1),
+		BPF_EXIT_INSN()
+	};
+
+	probe_misc_feature(insns, ARRAY_SIZE(insns),
+			   define_prefix, ifindex,
+			   "have_v3_isa_extension",
+			   "ISA extension v3",
+			   "V3_ISA_EXTENSION");
+}
+
 static void
 section_system_config(enum probe_component target, const char *define_prefix)
 {
@@ -823,6 +865,8 @@ static void section_misc(const char *def
 			    define_prefix);
 	probe_large_insn_limit(define_prefix, ifindex);
 	probe_bounded_loops(define_prefix, ifindex);
+	probe_v2_isa_extension(define_prefix, ifindex);
+	probe_v3_isa_extension(define_prefix, ifindex);
 	print_end_section();
 }