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

From: Lai Jiangshan
Date: Mon Jun 28 2010 - 03:58:50 EST

H. Peter Anvin wrote:
> 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...

There is alternative in use_xsave().
use_xsave() should return false in my system, but it returns true after this patch applied.

>> Does this patch change the return value of "use_xsave()"
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