On Thu, Apr 17, 2025 at 12:14 AM Alexis Lothoré alexis.lothore@bootlin.com wrote:
Hi Andrii,
On Wed Apr 16, 2025 at 11:24 PM CEST, Andrii Nakryiko wrote:
On Fri, Apr 11, 2025 at 1:32 PM Alexis Lothoré (eBPF Foundation) alexis.lothore@bootlin.com wrote:
In order to properly JIT the trampolines needed to attach BPF programs to functions, some architectures like ARM64 need to know about the alignment needed for the function arguments. Such alignment can generally be deduced from the argument size, but that's not completely true for composite types. In the specific case of ARM64, the AAPCS64 ABI defines that a composite type which needs to be passed through stack must be aligned on the maximum between 8 and the largest alignment constraint of its first-level members. So the JIT compiler needs more information about the arguments to make sure to generate code that respects those alignment constraints.
For struct arguments, add information about the size of the largest first-level member in the struct btf_func_model to allow the JIT compiler to guess the needed alignment. The information is quite
I might be missing something, but how can the *size* of the field be used to calculate that argument's *alignment*? i.e., I don't understand why arg_largest_member_size needs to be calculated instead of arg_largest_member_alignment...
Indeed I initially checked whether I could return directly some alignment info from btf, but it then involves the alignment computation in the btf module. Since there could be minor differences between architectures about alignment requirements, I though it would be better to in fact keep alignment computation out of the btf module. For example, I see that 128 bits values are aligned on 16 bytes on ARM64, while being aligned on 8 bytes on S390.
And since for ARM64, all needed alignments are somehow derived from size (it is either directly size for fundamental types, or alignment of the largest member for structs, which is then size of largest member), returning the size seems to be enough to allow the JIT side to compute alignments.
If you mean the size of "primitive" field and/or array element (applied recursively for all embedded structs/unions) then yes, that's close enough. But saying just "largest struct member" is wrong, because for
struct blah { struct { int whatever[128]; } heya; };
blah.heya has a large size, but alignment is still just 4 bytes.
I'd suggest looking at btf__align_of() in libbpf (tools/lib/bpf/btf.c) to see how we calculate alignment there. It seems to work decently enough. It won't cover any arch-specific extra rules like double needing 16-byte alignment (I vaguely remember something like that for some architectures, but I might be misremembering), or anything similar. It also won't detect (I don't think it's possible without DWARF) artificially increased alignment with attribute((aligned(N))).
specific, but it allows to keep arch-specific concerns (ie: guessing the final needed alignment for an argument) isolated in each JIT compiler.
couldn't all this information be calculated in the JIT compiler (if JIT needs that) from BTF?
From what I understand, the JIT compiler does not have access to BTF info, only a substract from it, arranged in a struct btf_func_model ? This struct btf_func_model already has size info for standard types, but for structs we need some additional info about the members, hence this arg_largest_member_alignment addition in btf_func_model.
Thanks,
Alexis
-- Alexis Lothoré, Bootlin Embedded Linux and Kernel engineering https://bootlin.com