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 10:24:15 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Juergen Gross <jgross@...e.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


> +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.

-boris