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 13:56:54 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Andrew Cooper <andrew.cooper3@...rix.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 12:02, Andrew Cooper wrote:
> 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."

I'll update the comment.

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

Thanks,

Juergen