Re: [RFC] x86/csum: rewrite csum_partial()

On 11/11/2021 16:52, Eric Dumazet wrote:

On Thu, Nov 11, 2021 at 8:02 AM Eric Dumazet [off-list ref] wrote:

quoted

Thanks Peter !

This is more or less the first version I wrote. (I was doing tests for
(len & 32), (len & 16) .. to not have to update len in these blocks.

Then, I tried to add an inline version, a la ip_fast_csum() but for IPv6.

Then I came up with the version I sent, for some reason my .config had
temporarily disabled CONFIG_RETPOLINE,
thanks for reminding me this !

I also missed this warning anyway :
arch/x86/lib/csum-partial_64.o: warning: objtool: csum_partial()+0x2f:
unannotated intra-function call

I will spend a bit more time on this before sending a V2, thanks again !

BTW, I could not understand why :

               result = add32_with_carry(result, *(u32 *)buff);

generates this code :

 123: 41 8b 09              mov    (%r9),%ecx
 126: 89 4d f8              mov    %ecx,-0x8(%rbp)
 129: 03 45 f8              add    -0x8(%rbp),%eax
 12c: 83 d0 00              adc    $0x0,%eax

Are you using Clang?  There is a long outstanding code generation bug
where an "rm" constraint is converted to "m" internally.

https://bugs.llvm.org/show_bug.cgi?id=47530

Even a stopgap of pretending "rm" means "r" would result in far better
code, 99% of the time.

Apparently add32_with_carry() forces the use of use of a temporary in memory

While
               asm("   addl 0*4(%[src]),%[res]\n"
                   "   adcl $0,%[res]\n"
                       : [res] "=r" (result)
                       : [src] "r" (buff), "[res]" (result)
                        : "memory");

Just as a minor note about the asm constraints here and elsewhere

    : [res] "=r" (result)
    : "res" (result)

ought to be just [res] "+r" (result).  The result variable really is
read and written by the asm fragments.

~Andrew