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

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Message-ID: <e60e4d87-8589-f22e-a5d0-a2d4e4b81d1c@suse.com>
Date:   Mon, 24 Apr 2017 08:23:09 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Andrew Cooper <andrew.cooper3@...rix.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 16:51, Boris Ostrovsky wrote:
> On 04/21/2017 10:45 AM, Andrew Cooper wrote:
>> 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
> 
> It's exception fixup that is not set up yet --- we are executing here
> before "main" kernel's entry point.
> 
> This is somewhat similar to what
> arch/x86/kernel/head_64.S:early_idt_handler_common() does --- it has
> special handling for early fixup --- early_fixup_exception().
> 
> I wonder though --- can this feature masking be deferred until a bit later?

At least the xsave feature is tested rather early: it is needed in
early_cpu_init() being called way before trap_init().


Juergen