Re: x86/csum: Remove unnecessary odd handling

[Eric Dumazet <edumazet@google.com>]

On Sat, Jan 6, 2024 at 1:19 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Fri, 5 Jan 2024 at 15:53, David Laight <David.Laight@aculab.com> wrote:
> >
> > I'd have to fix his benchmark code first :-)
> > You can't use the TSC unless you lock the cpu frequency.
> > The longer the test runs for the faster the cpu will run.
>
> They'll stabilize, it has soem cycle result aging code.
>
> But yes, set the CPU policy to 'performance' or do performance
> counters if you care deeply.
>
> > On a related point, do you remember what the 'killer app'
> > was for doing the checksum in copy_to/from_user?
>
> No. It's a long time ago, and many things have changed since.
>
> It's possible the copy-and-csum it's not worth it any more, simply
> because all modern network cards will do the csum for us, and I think
> loopback sets a flag saying "no need to checksum" too.
>
> But I do have a strong memory of it being a big deal back when. A
> _loong_ time ago.
>

On a related note, at least with clang, I found that csum_ipv6_magic()
is needlessly using temporary on-stack storage,
showing a stall on Cascade Lake unless I am patching add32_with_carry() :

diff --git a/arch/x86/include/asm/checksum_64.h
b/arch/x86/include/asm/checksum_64.h
index 407beebadaf45a748f91a36b78bd1d023449b132..c3d6f47626c70d81f0c2ba401d85050b09a39922
100644
--- a/arch/x86/include/asm/checksum_64.h
+++ b/arch/x86/include/asm/checksum_64.h
@@ -171,7 +171,7 @@ static inline unsigned add32_with_carry(unsigned
a, unsigned b)
        asm("addl %2,%0\n\t"
            "adcl $0,%0"
            : "=r" (a)
-           : "0" (a), "rm" (b));
+           : "0" (a), "r" (b));
        return a;
 }

Before patch :

ffffffff81b9b600 <csum_ipv6_magic>:
ffffffff81b9b600: 0f 1f 44 00 00        nopl   0x0(%rax,%rax,1)
ffffffff81b9b601: R_X86_64_NONE __fentry__-0x4
ffffffff81b9b605: 55                    push   %rbp
ffffffff81b9b606: 48 89 e5              mov    %rsp,%rbp
ffffffff81b9b609: 48 83 ec 04          sub    $0x4,%rsp     // This
will be removed after patch
ffffffff81b9b60d: 0f ca                bswap  %edx
ffffffff81b9b60f: 89 c9                mov    %ecx,%ecx
ffffffff81b9b611: 48 c1 e1 08          shl    $0x8,%rcx
ffffffff81b9b615: 44 89 c0              mov    %r8d,%eax
ffffffff81b9b618: 48 01 d0              add    %rdx,%rax
ffffffff81b9b61b: 48 01 c8              add    %rcx,%rax
ffffffff81b9b61e: 48 03 07              add    (%rdi),%rax
ffffffff81b9b621: 48 13 47 08          adc    0x8(%rdi),%rax
ffffffff81b9b625: 48 13 06              adc    (%rsi),%rax
ffffffff81b9b628: 48 13 46 08          adc    0x8(%rsi),%rax
ffffffff81b9b62c: 48 83 d0 00          adc    $0x0,%rax
ffffffff81b9b630: 48 89 c1              mov    %rax,%rcx
ffffffff81b9b633: 48 c1 e9 20          shr    $0x20,%rcx
ffffffff81b9b637: 89 4d fc              mov    %ecx,-0x4(%rbp)  //
Store exc on the stack
ffffffff81b9b63a: 03 45 fc              add    -0x4(%rbp),%eax  //
reads the value right away, stalling some Intel cpus.
ffffffff81b9b63d: 83 d0 00              adc    $0x0,%eax
ffffffff81b9b640: 89 c1                mov    %eax,%ecx   // Note that
ecs content was scratch anyway
ffffffff81b9b642: c1 e1 10              shl    $0x10,%ecx
ffffffff81b9b645: 25 00 00 ff ff        and    $0xffff0000,%eax
ffffffff81b9b64a: 01 c8                add    %ecx,%eax
ffffffff81b9b64c: 15 ff ff 00 00        adc    $0xffff,%eax
ffffffff81b9b651: f7 d0                not    %eax
ffffffff81b9b653: c1 e8 10              shr    $0x10,%eax
ffffffff81b9b656: 48 83 c4 04          add    $0x4,%rsp
ffffffff81b9b65a: 5d                    pop    %rbp
ffffffff81b9b65b: c3                    ret
ffffffff81b9b65c: cc                    int3
ffffffff81b9b65d: 00 00                add    %al,(%rax)
ffffffff81b9b65f: cc                    int3

After patch:

00000000000000a0 <csum_ipv6_magic>:
  a0: 0f 1f 44 00 00        nopl   0x0(%rax,%rax,1)
a1: R_X86_64_NONE __fentry__-0x4
  a5: 55                    push   %rbp
  a6: 48 89 e5              mov    %rsp,%rbp
  a9: 0f ca                bswap  %edx
  ab: 89 c9                mov    %ecx,%ecx
  ad: 48 c1 e1 08          shl    $0x8,%rcx
  b1: 44 89 c0              mov    %r8d,%eax
  b4: 48 01 d0              add    %rdx,%rax
  b7: 48 01 c8              add    %rcx,%rax
  ba: 48 03 07              add    (%rdi),%rax
  bd: 48 13 47 08          adc    0x8(%rdi),%rax
  c1: 48 13 06              adc    (%rsi),%rax
  c4: 48 13 46 08          adc    0x8(%rsi),%rax
  c8: 48 83 d0 00          adc    $0x0,%rax
  cc: 48 89 c1              mov    %rax,%rcx
  cf: 48 c1 e9 20          shr    $0x20,%rcx
  d3: 01 c8                add    %ecx,%eax   // just better !
  d5: 83 d0 00              adc    $0x0,%eax
  d8: 89 c1                mov    %eax,%ecx
  da: c1 e1 10              shl    $0x10,%ecx
  dd: 25 00 00 ff ff        and    $0xffff0000,%eax
  e2: 01 c8                add    %ecx,%eax
  e4: 15 ff ff 00 00        adc    $0xffff,%eax
  e9: f7 d0                not    %eax
  eb: c1 e8 10              shr    $0x10,%eax
  ee: 5d                    pop    %rbp
  ef: c3                    ret
  f0: cc                    int3