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SubjectRe: TCP Segmentation Offloading (TSO)
From
Hello!

kuznet> > [1] Kudos to
kuznet>
kuznet> Hmm... wait awhile with celebrating, the implementation in tcp is still
kuznet> at level of a toy. Well, and it happens to crash, the patch is enclosed.

I guess it may also depend on bad implementations of csum_partial().
It's wrong that some architecture assume every data in a skbuff are
aligned on a 2byte boundary so that it would access a byte next to
the the last byte where no pages might be there.

And we should know sendfile systemcall also pass pages with any offsets
and any byte while csum_partial() may be called from everywhere
in the kernel including device drivers.

It's time to fix csum_partial().


P.S.
Using "bswap" is little bit tricky.

Regards,
Hirokazu Takahashi
VP Engineering Dept.
VA Linux Systems Japan



--- linux/arch/i386/lib/checksum.S.BUG Sun Sep 1 17:00:59 2030
+++ linux/arch/i386/lib/checksum.S Mon Sep 2 13:09:09 2030
@@ -126,8 +126,8 @@ csum_partial:
movl 16(%esp),%ecx # Function arg: int len
movl 12(%esp),%esi # Function arg: const unsigned char *buf

- testl $2, %esi
- jnz 30f
+ testl $3, %esi
+ jnz 25f
10:
movl %ecx, %edx
movl %ecx, %ebx
@@ -145,6 +145,20 @@ csum_partial:
lea 2(%esi), %esi
adcl $0, %eax
jmp 10b
+25:
+ testl $1, %esi
+ jz 30f
+ # buf is odd
+ dec %ecx
+ jl 90f
+ bswap %eax
+ movzbl (%esi), %ebx
+ shll $8, %ebx
+ addl %ebx, %eax
+ adcl $0, %eax
+ inc %esi
+ testl $2, %esi
+ jz 10b

30: subl $2, %ecx
ja 20b
@@ -211,6 +225,10 @@ csum_partial:
addl %ebx,%eax
adcl $0,%eax
80:
+ testl $1, 12(%esp)
+ jz 90f
+ bswap %eax
+90:
popl %ebx
popl %esi
ret
-
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