On Fri, 2011-09-09 at 03:41 +0100, Ben Hutchings wrote:
> On Thu, 2011-09-08 at 17:46 -0700, Andres Salomon wrote:
> > Package: linux-image-3.1.0-rc4-486
> > Version: 3.1.0~rc4-1~experimental.1
> > Severity: normal
> >
> > During boot on an OLPC XO-1, I encounter the following problem. I
> > believe it's the statement BUG_ON(a->cpuid >= NCAPINTS*32), though I'm
> > not staring at an exact copy of linux-2.6 source.
> [...]
>
> Agreed.
>
> Variable a is in the EBX register, value 0xc143c668. Looking at a dump
> of this kernel image, the __alt_instructions array starts with:
>
> c143c638 7e51bcff 1c050000 1a000503 7d51bcff ~Q..........}Q..
> c143c648 13050000 1a000503 6653bcff 0a050000 ........fS......
> c143c658 19000503 00000000 ce55bcff 00000000 .........U......
> c143c668 9a000200 cf55bcff f1040000 e4000303 .....U..........
> ...
>
> Converting back to C structures:
>
> /* instr_offset, repl_offset, cpuid, instrlen, replacementlen */
> { 0xffbc517e, 0x0000051c, 0x001a, 0x05, 0x03 },
> { 0xffbc517d, 0x00000513, 0x001a, 0x05, 0x03 },
> { 0xffbc5366, 0x0000050a, 0x0019, 0x05, 0x03 },
> { 0x00000000, 0xffbc55ce, 0x0000, 0x00, 0x00 },
> { 0x0002009a, 0xffbc55cf, 0x04f1, 0x00, 0x00 },
> ...
>
> Oops, we have 4 bytes of zeroes between what should be elements 2 and 3.
> That results in __alt_instructions[4].cpuid == 0x04f1 which trips the
> BUG check. This is nothing OLPC-specific; it's going to happen on any
> machine trying to boot this image.
>
> So the question is, where does that padding come from?Element 2 is for the wmb() in async_hcall() in arch/x86/lguest/boot.c. Element 3 is for the use_xsave() in __unlazy_fpu(). There is no padding between any of the other elements in __alt_instructions in this image. But there's nothing very special or obviously wrong about those two uses of alternatives. Ben. -- Ben Hutchings I say we take off; nuke the site from orbit. It's the only way to be sure.
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