Re: [RFC bpf-next v1 0/8] no_caller_saved_registers attribute for helper calls

Andrii Nakryiko
2024-07-02T00:41:40Z

On Sat, Jun 29, 2024 at 2:48 AM Eduard Zingerman <eddyz87@gmail.com> wrote:
>
> This RFC seeks to allow using no_caller_saved_registers gcc/clang
> attribute with some BPF helper functions (and kfuncs in the future).
>
> As documented in [1], this attribute means that function scratches
> only some of the caller saved registers defined by ABI.
> For BPF the set of such registers could be defined as follows:
> - R0 is scratched only if function is non-void;
> - R1-R5 are scratched only if corresponding parameter type is defined
> in the function prototype.
>
> The goal of the RFC is to implement no_caller_saved_registers
> (nocsr for short) in a backwards compatible manner:
> - for kernels that support the feature, gain some performance boost
> from better register allocation;
> - for kernels that don't support the feature, allow programs execution
> with minor performance losses.
>
> To achieve this, use a scheme suggested by Alexei Starovoitov:
> - for nocsr calls clang allocates registers as-if relevant r0-r5
> registers are not scratched by the call;
> - as a post-processing step, clang visits each nocsr call and adds
> spill/fill for every live r0-r5;
> - stack offsets used for spills/fills are allocated as minimal
> stack offsets in whole function and are not used for any other
> purposes;
> - when kernel loads a program, it looks for such patterns
> (nocsr function surrounded by spills/fills) and checks if
> spill/fill stack offsets are used exclusively in nocsr patterns;
> - if so, and if current JIT inlines the call to the nocsr function

JIT inlines or BPF verifier can inline as well?

> (e.g. a helper call), kernel removes unnecessary spill/fill pairs;
> - when old kernel loads a program, presence of spill/fill pairs
> keeps BPF program valid, albeit slightly less efficient.
>
> Corresponding clang/llvm changes are available in [2].
>
> The patch-set uses bpf_get_smp_processor_id() function as a canary,
> making it the first helper with nocsr attribute.
>
> For example, consider the following program:
>
> #define __no_csr __attribute__((no_caller_saved_registers))
> #define SEC(name) __attribute__((section(name), used))
> #define bpf_printk(fmt, ...) bpf_trace_printk((fmt), sizeof(fmt), __VA_ARGS__)
>
> typedef unsigned int __u32;
>
> static long (* const bpf_trace_printk)(const char *fmt, __u32 fmt_size, ...) = (void *) 6;
> static __u32 (*const bpf_get_smp_processor_id)(void) __no_csr = (void *)8;
>
> SEC("raw_tp")
> int test(void *ctx)
> {
> __u32 task = bpf_get_smp_processor_id();
> bpf_printk("ctx=%p, smp=%d", ctx, task);
> return 0;
> }
>
> char _license[] SEC("license") = "GPL";
>
> Compiled (using [2]) as follows:
>
> $ clang --target=bpf -O2 -g -c -o nocsr.bpf.o nocsr.bpf.c
> $ llvm-objdump --no-show-raw-insn -Sd nocsr.bpf.o
> ...
> 3rd parameter for printk call removable spill/fill pair
> .--- 0: r3 = r1 |
> ; | __u32 task = bpf_get_smp_processor_id(); |
> | 1: *(u64 *)(r10 - 0x8) = r3 <----------|
> | 2: call 0x8 |
> | 3: r3 = *(u64 *)(r10 - 0x8) <----------'
> ; | bpf_printk("ctx=%p, smp=%d", ctx, task);
> | 4: r1 = 0x0 ll
> | 6: r2 = 0xf
> | 7: r4 = r0
> '--> 8: call 0x6
> ; return 0;
> 9: r0 = 0x0
> 10: exit
>
> Here is how the program looks after verifier processing:
>
> # bpftool prog load ./nocsr.bpf.o /sys/fs/bpf/nocsr-test
> # bpftool prog dump xlated pinned /sys/fs/bpf/nocsr-test
> int test(void * ctx):
> ; int test(void *ctx)
> 0: (bf) r3 = r1 <--------- 3rd printk parameter
> ; __u32 task = bpf_get_smp_processor_id();
> 1: (b4) w0 = 197132 <--------- inlined helper call,
> 2: (bf) r0 = r0 <--------- spill/fill pair removed
> 3: (61) r0 = *(u32 *)(r0 +0) <---------
> ; bpf_printk("ctx=%p, smp=%d", ctx, task);
> 4: (18) r1 = map[id:13][0]+0
> 6: (b7) r2 = 15
> 7: (bf) r4 = r0
> 8: (85) call bpf_trace_printk#-125920
> ; return 0;
> 9: (b7) r0 = 0
> 10: (95) exit
>
> [1] https://clang.llvm.org/docs/AttributeReference.html#no-caller-saved-registers
> [2] https://github.com/eddyz87/llvm-project/tree/bpf-no-caller-saved-registers
>
> Eduard Zingerman (8):
> bpf: add a get_helper_proto() utility function
> bpf: no_caller_saved_registers attribute for helper calls
> bpf, x86: no_caller_saved_registers for bpf_get_smp_processor_id()
> selftests/bpf: extract utility function for BPF disassembly
> selftests/bpf: no need to track next_match_pos in struct test_loader
> selftests/bpf: extract test_loader->expect_msgs as a data structure
> selftests/bpf: allow checking xlated programs in verifier_* tests
> selftests/bpf: test no_caller_saved_registers spill/fill removal
>
> include/linux/bpf.h | 6 +
> include/linux/bpf_verifier.h | 9 +
> kernel/bpf/helpers.c | 1 +
> kernel/bpf/verifier.c | 346 +++++++++++++-
> tools/testing/selftests/bpf/Makefile | 1 +
> tools/testing/selftests/bpf/disasm_helpers.c | 50 ++
> tools/testing/selftests/bpf/disasm_helpers.h | 12 +
> .../selftests/bpf/prog_tests/ctx_rewrite.c | 71 +--
> .../selftests/bpf/prog_tests/verifier.c | 7 +
> tools/testing/selftests/bpf/progs/bpf_misc.h | 6 +
> .../selftests/bpf/progs/verifier_nocsr.c | 437 ++++++++++++++++++
> tools/testing/selftests/bpf/test_loader.c | 170 +++++--
> tools/testing/selftests/bpf/test_progs.h | 1 -
> tools/testing/selftests/bpf/testing_helpers.c | 1 +
> 14 files changed, 986 insertions(+), 132 deletions(-)
> create mode 100644 tools/testing/selftests/bpf/disasm_helpers.c
> create mode 100644 tools/testing/selftests/bpf/disasm_helpers.h
> create mode 100644 tools/testing/selftests/bpf/progs/verifier_nocsr.c
>
> --
> 2.45.2
>