| commit | 64e464349bfca0d90e07f6db2f710d4d53cdacd4 | [log] [tgz] |
|---|---|---|
| author | eddyz87 <eddyz87@gmail.com> | Thu Aug 22 03:40:56 2024 +0300 |
| committer | GitHub <noreply@github.com> | Thu Aug 22 03:40:56 2024 +0300 |
| tree | 8aadabfb2e28f52b84fc9c9104db337c767063f5 | |
| parent | 04c827d0b5e629ba53e8ede94811a13a96db36a4 [diff] |
[BPF] introduce __attribute__((bpf_fastcall)) (#105417)
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.
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