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

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Date:   Fri, 21 Apr 2017 15:45:40 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        <linux-kernel@...r.kernel.org>, <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH v3 09/11] x86/xen: use capabilities instead of fake cpuid
 values for xsave

On 21/04/17 15:38, Juergen Gross wrote:
> 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?

Hmm fine.  The status quo is probably best then to unblock this series.

As an independent question, why are exceptions set up so late?  They
really should be the very first thing done.

~Andrew