On 4/24/2025 3:24 AM, Alexis Lothoré wrote:
Hi Andrii,
On Wed Apr 23, 2025 at 7:15 PM CEST, Andrii Nakryiko wrote:
On Thu, Apr 17, 2025 at 12:14 AM Alexis Lothoré
<alexis.loth...@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.loth...@bootlin.com> wrote:
[...]
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.
Indeed, that's another case making my proposal fail :)
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))).
Thanks for the pointer, I'll take a look at it. The more we discuss this
series, the less member size sounds relevant for what I'm trying to achieve
here.
Following Xu's comments, I have been thinking about how I could detect the
custom alignments and packing on structures, and I was wondering if I could
somehow benefit from __attribute__ encoding in BTF info ([1]). But
following your hint, I also see some btf_is_struct_packed() in
tools/lib/bpf/btf_dump.c that could help. I'll dig this further and see if
I can manage to make something work with all of this.
With DWARF info, we might not need to detect the structure alignment anymore,
since the DW_AT_location attribute tells us where the structure parameter is
located on the stack, and DW_AT_byte_size gives us the size of the structure.
Thanks,
Alexis
[1]
https://lore.kernel.org/bpf/20250130201239.1429648-1-ihor.solod...@linux.dev/
