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Message-ID: <20200930172317.48f85a5b@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date: Wed, 30 Sep 2020 17:23:17 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Andrew Lunn <andrew@...n.ch>
Cc: Johannes Berg <johannes@...solutions.net>,
Jiri Pirko <jiri@...nulli.us>,
Michal Kubecek <mkubecek@...e.cz>, dsahern@...nel.org,
pablo@...filter.org, netdev@...r.kernel.org
Subject: Re: Genetlink per cmd policies
On Thu, 1 Oct 2020 01:38:17 +0200 Andrew Lunn wrote:
> > > > +static void genl_op_from_full(const struct genl_family *family,
> > > > + unsigned int i, struct genl_ops *op)
> > > > +{
> > > > + memcpy(op, &family->ops[i], sizeof(*op));
> > >
> > > What's wrong with struct assignment? :)
> > >
> > > *op = family->ops[i];
> >
> > Code size :)
> >
> > text data bss dec hex
> > 22657 3590 64 26311 66c7 memcpy
> > 23103 3590 64 26757 6885 struct
>
> You might want to show that to the compiler people. Did you look at
> the assembly?
Somewhere along the line I lost the ability to decipher compiler code :(
Two snippets follow (both with -Os to prevent inlining)
Assignment:
static void genl_op_from_full(const struct genl_family *family,
unsigned int i, struct genl_ops *op)
{
0: e8 00 00 00 00 callq 5 <genl_op_from_full+0x5>
5: 41 54 push %r12
7: 49 89 d4 mov %rdx,%r12
a: 55 push %rbp
b: 89 f5 mov %esi,%ebp
d: 53 push %rbx
e: 48 89 fb mov %rdi,%rbx
*op = family->ops[i];
11: 48 6b ed 30 imul $0x30,%rbp,%rbp
15: 48 83 c7 40 add $0x40,%rdi
19: e8 00 00 00 00 callq 1e <genl_op_from_full+0x1e>
1e: 48 03 6b 40 add 0x40(%rbx),%rbp
22: be 30 00 00 00 mov $0x30,%esi
27: 4c 89 e7 mov %r12,%rdi
2a: e8 00 00 00 00 callq 2f <genl_op_from_full+0x2f>
2f: be 30 00 00 00 mov $0x30,%esi
34: 48 89 ef mov %rbp,%rdi
37: e8 00 00 00 00 callq 3c <genl_op_from_full+0x3c>
3c: b9 0c 00 00 00 mov $0xc,%ecx
41: 4c 89 e7 mov %r12,%rdi
44: 48 89 ee mov %rbp,%rsi
47: f3 a5 rep movsl %ds:(%rsi),%es:(%rdi)
if (!op->maxattr)
49: 49 8d 7c 24 28 lea 0x28(%r12),%rdi
4e: e8 00 00 00 00 callq 53 <genl_op_from_full+0x53>
53: 41 83 7c 24 28 00 cmpl $0x0,0x28(%r12)
59: 75 11 jne 6c <genl_op_from_full+0x6c>
op->maxattr = family->maxattr;
5b: 48 8d 7b 1c lea 0x1c(%rbx),%rdi
5f: e8 00 00 00 00 callq 64 <genl_op_from_full+0x64>
64: 8b 43 1c mov 0x1c(%rbx),%eax
67: 41 89 44 24 28 mov %eax,0x28(%r12)
Memcpy:
00000000000001a2 <genl_op_from_full>:
{
1a2: e8 00 00 00 00 callq 1a7 <genl_op_from_full+0x5>
1a7: 41 54 push %r12
1a9: 49 89 fc mov %rdi,%r12
memcpy(op, &family->ops[i], sizeof(*op));
1ac: 48 83 c7 40 add $0x40,%rdi
{
1b0: 55 push %rbp
1b1: 89 f5 mov %esi,%ebp
1b3: 53 push %rbx
1b4: 48 89 d3 mov %rdx,%rbx
memcpy(op, &family->ops[i], sizeof(*op));
1b7: e8 00 00 00 00 callq 1bc <genl_op_from_full+0x1a>
1bc: 89 ee mov %ebp,%esi
if (q_size < size)
__read_overflow2();
}
if (p_size < size || q_size < size)
fortify_panic(__func__);
return __underlying_memcpy(p, q, size);
1be: ba 30 00 00 00 mov $0x30,%edx
1c3: 48 89 df mov %rbx,%rdi
1c6: 48 6b f6 30 imul $0x30,%rsi,%rsi
1ca: 49 03 74 24 40 add 0x40(%r12),%rsi
1cf: e8 00 00 00 00 callq 1d4 <genl_op_from_full+0x32>
if (!op->maxattr)
1d4: 48 8d 7b 28 lea 0x28(%rbx),%rdi
1d8: e8 00 00 00 00 callq 1dd <genl_op_from_full+0x3b>
1dd: 83 7b 28 00 cmpl $0x0,0x28(%rbx)
1e1: 75 12 jne 1f5 <genl_op_from_full+0x53>
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