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

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Date:   Tue, 18 Apr 2017 11:02:03 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Juergen Gross <jgross@...e.com>, <linux-kernel@...r.kernel.org>,
        <xen-devel@...ts.xenproject.org>
CC:     <boris.ostrovsky@...cle.com>
Subject: Re: [Xen-devel] [PATCH v3 09/11] x86/xen: use capabilities instead of
 fake cpuid values for xsave

On 18/04/17 07:31, Juergen Gross wrote:
> @@ -281,22 +274,24 @@ static bool __init xen_check_mwait(void)
>  	return false;
>  #endif
>  }
> -static void __init xen_init_cpuid_mask(void)
> +
> +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. */

The code is fine, but this comment isn't going to be any help to people
reading this code in 6 months time.

How about this:

"Xen 4.0 and older accidentally leaked the host XSAVE flag into guest
view, despite not being able to support guests using the functionality. 
Probe for the actual availability of XSAVE by seeing whether xgetbv
executes successfully or raises #UD."

Everything else is fine, so Reviewed-by: Andrew Cooper
<andrew.cooper3@...rix.com>

~Andrew