| Message ID | 20240629094733.3863850-1-eddyz87@gmail.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | no_caller_saved_registers attribute for helper calls | expand |
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 >
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 (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