Re: [tip:x86/alternatives] x86, alternatives: Use 16-bit numbersfor cpufeature index

From: H. Peter Anvin
Date: Fri Jun 25 2010 - 11:35:53 EST

On 06/25/2010 02:20 AM, Lai Jiangshan wrote:
>> x86, alternatives: Use 16-bit numbers for cpufeature index
>> We already have cpufeature indicies above 255, so use a 16-bit number
>> for the alternatives index. This consumes a padding field and so
>> doesn't add any size, but it means that abusing the padding field to
>> create assembly errors on overflow no longer works. We can retain the
>> test simply by redirecting it to the .discard section, however.
> My machine hits "invalid opcode" at *prepare_to_copy+0x79,
> and it can't boot up.
> (gdb) l *prepare_to_copy+0x79
> 0xc0101789 is in prepare_to_copy (/home/njubee/work/linux-2.6-tip/arch/x86/include/asm/xsave.h:118).
> 113
> 114 static inline void fpu_xsave(struct fpu *fpu)
> 115 {
> 116 /* This, however, we can work around by forcing the compiler to select
> 117 an addressing mode that doesn't require extended registers. */
> 118 __asm__ __volatile__(".byte " REX_PREFIX "0x0f,0xae,0x27"
> 119 : : "D" (&(fpu->state->xsave)),
> 120 "a" (-1), "d"(-1) : "memory");
> 121 }
> 122 #endif

There are no alternatives in that code, at all... so it makes me really
wonder what is going on. One possibility, of course, is that one
alternative has ended up with the wrong address. Will look...


H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.

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