Subject: Re: [PATCH v3 09/11] x86/xen: use capabilities instead of fake cpuid
 values for xsave
To: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org,
        Andrew Cooper <andrew.cooper3@citrix.com>
References: <20170418063119.11654-1-jgross@suse.com>
 <20170418063119.11654-10-jgross@suse.com>
 <da6c4270-2123-5203-4e76-32f4b616242f@oracle.com>
From: Juergen Gross <jgross@suse.com>
Message-ID: <12028326-0f19-9430-d8b7-a376facf8749@suse.com>
Date: Fri, 21 Apr 2017 16:38:42 +0200
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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