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

From	Andrew Cooper <andrew.cooper3@citrix.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH v3 09/11] x86/xen: use capabilities instead of fake cpuid values for xsave
Date	2017-04-21 20:30 +0200
Message-ID	<tyLtF-Ix-45@gated-at.bofh.it> (permalink)
References	<txuXT-2Q2-3@gated-at.bofh.it> <txuXU-2Q2-21@gated-at.bofh.it> <tyHJn-6SD-5@gated-at.bofh.it> <tyHT5-6W1-31@gated-at.bofh.it>
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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

Thread

[PATCH v3 09/11] x86/xen: use capabilities instead of fake cpuid values for xsave Juergen Gross <jgross@suse.com> - 2017-04-18 08:40 +0200
  Re: [Xen-devel] [PATCH v3 09/11] x86/xen: use capabilities instead of  fake cpuid values for xsave Andrew Cooper <andrew.cooper3@citrix.com> - 2017-04-18 12:10 +0200
    Re: [Xen-devel] [PATCH v3 09/11] x86/xen: use capabilities instead of  fake cpuid values for xsave Juergen Gross <jgross@suse.com> - 2017-04-18 14:00 +0200
  Re: [PATCH v3 09/11] x86/xen: use capabilities instead of fake cpuid  values for xsave Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-04-21 16:30 +0200
    Re: [PATCH v3 09/11] x86/xen: use capabilities instead of fake cpuid  values for xsave Juergen Gross <jgross@suse.com> - 2017-04-21 16:40 +0200
      Re: [PATCH v3 09/11] x86/xen: use capabilities instead of fake cpuid  values for xsave Andrew Cooper <andrew.cooper3@citrix.com> - 2017-04-21 20:30 +0200
        Re: [PATCH v3 09/11] x86/xen: use capabilities instead of fake cpuid  values for xsave Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-04-21 21:30 +0200
          Re: [PATCH v3 09/11] x86/xen: use capabilities instead of fake cpuid  values for xsave Juergen Gross <jgross@suse.com> - 2017-04-24 08:30 +0200

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