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Eduard Zingerman says:
====================
no_caller_saved_registers attribute for helper calls
This patch-set 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 patch-set 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 lowest
stack offsets in whole function and are not used for any other
purpose;
- 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):
0: (bf) r3 = r1 <--- 3rd printk parameter
; __u32 task = bpf_get_smp_processor_id();
1: (b4) w0 = 197324 <--. inlined helper call,
2: (bf) r0 = &(void __percpu *)(r0) <--- spill/fill
3: (61) r0 = *(u32 *)(r0 +0) <--' pair removed
; bpf_printk("ctx=%p, smp=%d", ctx, task);
4: (18) r1 = map[id:5][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
Change list:
- v3 -> v4:
- When nocsr spills/fills are removed in the subprogram, allow these
spills/fills to reside in [-MAX_BPF_STACK-48..MAX_BPF_STACK) range
(suggested by Alexei);
- Dropped patches with special handling for bpf_probe_read_kernel()
(requested by Alexei);
- Reset aux .nocsr_pattern and .nocsr_spills_num fields in
check_nocsr_stack_contract() (requested by Andrii).
Andrii, I have not added an additional flag to
struct bpf_subprog_info, it currently does not have holes
and I really don't like adding a bool field there just as an
alternative indicator that nocsr is disabled.
Indicator at the moment:
- nocsr_stack_off >= S16_MIN means that nocsr rewrite is enabled;
- nocsr_stack_off == S16_MIN means that nocsr rewrite is disabled.
- v2 -> v3:
- As suggested by Andrii, 'nocsr_stack_off' is no longer checked at
rewrite time, instead mark_nocsr_patterns() now does two passes
over BPF program:
- on a first pass it computes the lowest stack spill offset for
the subprogram;
- on a second pass this offset is used to recognize nocsr pattern.
- As suggested by Alexei, a new mechanic is added to work around a
situation mentioned by Andrii, when more helper functions are
marked as nocsr at compile time than current kernel supports:
- all {spill*,helper call,fill*} patterns are now marked as
insn_aux_data[*].nocsr_pattern, thus relaxing failure condition
for check_nocsr_stack_contract();
- spill/fill pairs are not removed for patterns where helper can't
be inlined;
- see mark_nocsr_pattern_for_call() for details an example.
- As suggested by Alexei, subprogram stack depth is now adjusted
if all spill/fill pairs could be removed. This adjustment has
to take place before optimize_bpf_loop(), hence the rewrite
is moved from do_misc_fixups() to remove_nocsr_spills_fills()
(again).
- As suggested by Andrii, special measures are taken to work around
bpf_probe_read_kernel() access to BPF stack, see patches 11, 12.
Patch #11 is very simplistic, a more comprehensive solution would
be to change the type of the third parameter of the
bpf_probe_read_kernel() from ARG_ANYTHING to something else and
not only check nocsr contract, but also propagate stack slot
liveness information. However, such change would require update in
struct bpf_call_arg_meta processing, which currently implies that
every memory parameter is followed by a size parameter.
I can work on these changes, please comment.
- Stylistic changes suggested by Andrii.
- Added acks from Andrii.
- Dropped RFC tag.
- v1 -> v2:
- assume that functions inlined by either jit or verifier
conform to no_caller_saved_registers contract (Andrii, Puranjay);
- allow nocsr rewrite for bpf_get_smp_processor_id()
on arm64 and riscv64 architectures (Puranjay);
- __arch_{x86_64,arm64,riscv64} macro for test_loader;
- moved remove_nocsr_spills_fills() inside do_misc_fixups() (Andrii);
- moved nocsr pattern detection from check_cfg() to a separate pass
(Andrii);
- various stylistic/correctness changes according to Andrii's
comments.
Revisions:
- v1 https://lore.kernel.org/bpf/20240629094733.3863850-1-eddyz87@gmail.com/
- v2 https://lore.kernel.org/bpf/20240704102402.1644916-1-eddyz87@gmail.com/
- v3 https://lore.kernel.org/bpf/20240715230201.3901423-1-eddyz87@gmail.com/
====================
Link: https://lore.kernel.org/r/20240722233844.1406874-1-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Linux kernel
============
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In order to build the documentation, use ``make htmldocs`` or
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