On Thu, Mar 21, 2024 at 11:05:01AM -0700, Andrii Nakryiko wrote: > Add arch-specific inlining of bpf_get_smp_processor_id() using x86-64's > gs segment-based addressing. > > Just to be on the safer side both rip-relative addressing is implemented > (providing a shorter instruction, but limiting offset to signed 32 bits) > and more universal absolute memory offset addressing is used as > a fallback in (unlikely) scenario that given offset doesn't fit int s32. > The latter is 5 bytes longer, and it seems compilers prefer rip-relative > instructions when compiling kernel code. > > Both instructions were tested and confirmed using gdb. We also already > have a BPF selftest (raw_tp_test_run) that validates correctness of > bpf_get_smp_processor_id(), while running target BPF program on each > online CPU. > > Here's a disassembly of bpf_get_smp_processor_id() helper: > > $ gdb -batch -ex 'file vmlinux' -ex 'set disassembly-flavor intel' -ex 'disassemble/r bpf_get_smp_processor_id' > Dump of assembler code for function bpf_get_smp_processor_id: > 0xffffffff810fa890 <+0>: 0f 1f 44 00 00 nop DWORD PTR [rax+rax*1+0x0] > 0xffffffff810fa895 <+5>: 65 8b 05 70 62 f3 7e mov eax,DWORD PTR gs:[rip+0x7ef36270] # 0x30b0c <pcpu_hot+12> > 0xffffffff810fa89c <+12>: 48 98 cdqe > 0xffffffff810fa89e <+14>: c3 ret > End of assembler dump. > > And here's a GDB disassembly dump of a piece of BPF program calling > bpf_get_smp_processor_id(). > > $ sudo cat /proc/kallsyms | rg 'pcpu_hot|bpf_prog_2b455b4f8a8d48c5_kexit' > 000000000002d840 A pcpu_hot > ffffffffa000f8a8 t bpf_prog_2b455b4f8a8d48c5_kexit [bpf] > > Then attaching GDB to the running kernel in QEMU and breaking inside BPF > program: > > (gdb) b *0xffffffffa000f8e2 > Breakpoint 1 at 0xffffffffa000f8e2 > > When RIP-relative instruction is used: > > 0xffffffffa000f8e2 mov %gs:0x6001df63(%rip),%eax # 0x2d84c <pcpu_hot+12> > 0xffffffffa000f8e9 cltq > > You can see that final address is resolved to <pcpu_hot+12> as expected. > > When absolute addressing is used: > > 0xffffffffa000f8e2 movabs %gs:0x2d84c,%eax > 0xffffffffa000f8ed cltq > > And here 0x2d84c matches pcpu_hot address from kallsyms (0x2d840), > plus 12 (0xc) bytes offset of cpu_number field. > > This inlining eliminates entire function call for this (rather trivial in terms > of instructions executed) helper, saving a bit of performance, but foremost > saving LBR records (1 for PERF_SAMPLE_BRANCH_ANY_RETURN mode, and 2 for > PERF_SAMPLE_BRANCH_ANY), which is what motivated this work in the first > place. this should also 'fix' the k[ret]probe-multi-fast benchmark issue right? https://lore.kernel.org/bpf/20240315051813.1320559-2-andrii@xxxxxxxxxx/ jirka > > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx> > --- > arch/x86/net/bpf_jit_comp.c | 26 +++++++++++++++++++++++++- > 1 file changed, 25 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c > index 4900b1ee019f..5b7fdc24b5b8 100644 > --- a/arch/x86/net/bpf_jit_comp.c > +++ b/arch/x86/net/bpf_jit_comp.c > @@ -457,6 +457,9 @@ static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf, > *pprog = prog; > } > > +/* reference to bpf_get_smp_processor_id() helper implementation to detect it for inlining */ > +extern u64 bpf_get_smp_processor_id(u64, u64, u64, u64, u64); > + > static int emit_patch(u8 **pprog, void *func, void *ip, u8 opcode) > { > u8 *prog = *pprog; > @@ -467,7 +470,28 @@ static int emit_patch(u8 **pprog, void *func, void *ip, u8 opcode) > pr_err("Target call %p is out of range\n", func); > return -ERANGE; > } > - EMIT1_off32(opcode, offset); > + > + /* inline bpf_get_smp_processor_id() to avoid calls */ > + if (opcode == 0xE8 && func == &bpf_get_smp_processor_id) { > + /* 7 to account for the mov instruction itself, > + * as rip value *after* mov instruction is used > + */ > + offset = (void *)&pcpu_hot.cpu_number - ip - 7; > + if (is_simm32(offset)) { > + /* mov eax,DWORD PTR gs:[rip+<offset>] ; <pcpu_hot+12> */ > + EMIT3_off32(0x65, 0x8b, 0x05, (u32)offset); > + } else { > + /* mov eax,DWORD PTR gs:<offset> ; <pcpu_hot+12> */ > + offset = (s64)(void *)&pcpu_hot.cpu_number; > + EMIT2(0x65, 0xa1); > + EMIT((u32)offset, 4); > + EMIT((u64)offset >> 32, 4); > + } > + EMIT2(0x48, 0x98); /* cdqe, zero-extend eax to rax */ > + } else { > + EMIT1_off32(opcode, offset); > + } > + > *pprog = prog; > return 0; > } > -- > 2.43.0 > >
