On Fri, 9 Apr 2021 22:44:50 +0200 Matteo Croce wrote:
> > What pops to mind (although quite nit picky) is the question if the
> > assembly changes much between driver which used to cache nr_frags and
> > now always going skb_shinfo(skb)->nr_frags? It's a relatively common
> > pattern.
>
> Since skb_shinfo() is a macro and skb_end_pointer() a static inline,
> it should be the same, but I was curious to check so, this is a diff
> between the following snippet before and afer the macro:
>
> int frags = skb_shinfo(skb)->nr_frags;
> int i;
> for (i = 0; i < frags; i++)
> kfree(skb->frags[i]);
>
> 1 file changed, 8 insertions(+), 7 deletions(-)
>
> --- ins1.s 2021-04-09 22:35:59.384523865 +0200
> +++ ins2.s 2021-04-09 22:36:08.132594737 +0200
> @@ -1,26 +1,27 @@
> iter:
> movsx rax, DWORD PTR [rdi+16]
> mov rdx, QWORD PTR [rdi+8]
> mov eax, DWORD PTR [rdx+rax]
> test eax, eax
> jle .L6
> push rbp
> - sub eax, 1
> + mov rbp, rdi
> push rbx
> - lea rbp, [rdi+32+rax*8]
> - lea rbx, [rdi+24]
> + xor ebx, ebx
> sub rsp, 8
> .L3:
> - mov rdi, QWORD PTR [rbx]
> - add rbx, 8
> + mov rdi, QWORD PTR [rbp+24+rbx*8]
> + add rbx, 1
> call kfree
> - cmp rbx, rbp
> - jne .L3
> + movsx rax, DWORD PTR [rbp+16]
> + mov rdx, QWORD PTR [rbp+8]
> + cmp DWORD PTR [rdx+rax], ebx
> + jg .L3
> add rsp, 8
> xor eax, eax
> pop rbx
> pop rbp
> ret
> .L6:
> xor eax, eax
> for (i = 0; i < frags; i++) ret
>
So looks like before compiler generated:
end = &frags[nfrags]
for (ptr = &frag[0]; ptr < end; ptr++)
and now it has to use the actual value of i, read nfrags in the loop
each time and compare it to i.
That makes sense, since it can't prove kfree() doesn't change nr_frags.
IDK if we care, but at least commit message should mention this.
