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This commit introduces attribute bpf_fastcall to declare BPF functions
that do not clobber some of the caller saved registers (R0-R5).
The idea is to generate the code complying with generic BPF ABI,
but allow compatible Linux Kernel to remove unnecessary spills and
fills of non-scratched registers (given some compiler assistance).
For such functions do register allocation as-if caller saved registers
are not clobbered, but later wrap the calls with spill and fill
patterns that are simple to recognize in kernel.
For example for the following C code:
#define __bpf_fastcall __attribute__((bpf_fastcall))
void bar(void) __bpf_fastcall;
void buz(long i, long j, long k);
void foo(long i, long j, long k) {
bar();
buz(i, j, k);
}
First allocate registers as if:
foo:
call bar # note: no spills for i,j,k (r1,r2,r3)
call buz
exit
And later insert spills fills on the peephole phase:
foo:
*(u64 *)(r10 - 8) = r1; # Such call pattern is
*(u64 *)(r10 - 16) = r2; # correct when used with
*(u64 *)(r10 - 24) = r3; # old kernels.
call bar
r3 = *(u64 *)(r10 - 24); # But also allows new
r2 = *(u64 *)(r10 - 16); # kernels to recognize the
r1 = *(u64 *)(r10 - 8); # pattern and remove spills/fills.
call buz
exit
The offsets for generated spills/fills are picked as minimal stack
offsets for the function. Allocated stack slots are not used for any
other purposes, in order to simplify in-kernel analysis.
15 lines
530 B
C
15 lines
530 B
C
// REQUIRES: bpf-registered-target
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// RUN: %clang_cc1 %s -triple bpf -verify
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__attribute__((bpf_fastcall)) int var; // expected-warning {{'bpf_fastcall' attribute only applies to functions and function pointers}}
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__attribute__((bpf_fastcall)) void func();
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__attribute__((bpf_fastcall(1))) void func_invalid(); // expected-error {{'bpf_fastcall' attribute takes no arguments}}
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void (*ptr1)(void) __attribute__((bpf_fastcall));
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void (*ptr2)(void);
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void foo(void) {
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ptr2 = ptr1; // not an error
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ptr1 = ptr2; // not an error
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}
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