Re: [Xen-devel] [PATCH v3 09/11] x86/xen: use capabilities instead of fake cpuid values for xsave

[Juergen Gross <jgross@xxxxxxxx>]

On 21/04/17 16:24, Boris Ostrovsky wrote:
> 
>> +static bool __init xen_check_xsave(void)
>>  {
>> -    unsigned int ax, bx, cx, dx;
>> -    unsigned int xsave_mask;
>> +    unsigned int err, eax, edx;
>>  
>> -    ax = 1;
>> -    cx = 0;
>> -    cpuid(1, &ax, &bx, &cx, &dx);
>> +    /* Test OSXSAVE capability via xgetbv instruction. */
>> +    asm volatile("1: .byte 0x0f,0x01,0xd0\n\t" /* xgetbv */
>> +                 "xor %[err], %[err]\n"
>> +                 "2:\n\t"
>> +                 ".pushsection .fixup,\"ax\"\n\t"
>> +                 "3: movl $1,%[err]\n\t"
>> +                 "jmp 2b\n\t"
>> +                 ".popsection\n\t"
>> +                 _ASM_EXTABLE(1b, 3b)
>> +                 : [err] "=r" (err), "=a" (eax), "=d" (edx)
>> +                 : "c" (0));
> 
> Have you tested this on processors where we actually trap on xgetbv?
> 
> I have an AMD box without XSAVE support and this is a fatal error. I
> suspect it's too early to use exception fixup framework here.

Uuh, too bad.

Then I fear we must use the other solution Andrew didn't like. :-(
Andrew, would you be okay with that?


Juergen


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