linux-kernel - Re: [ 174/262] x86, avx: dont use avx instructions with "noxsave" boot param

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Date:	Mon, 08 Oct 2012 07:55:31 +0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ben Hutchings <ben@...adent.org.uk>,
	Suresh Siddha <suresh.b.siddha@...el.com>
CC:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	alan@...rguk.ukuu.org.uk,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [ 174/262] x86, avx: dont use avx instructions with "noxsave" boot param

Userland should use a different query mechanism.  On the other hand, it will not hurt either.  On pre-xsave kernels AVX is a non-option.

Ben Hutchings <ben@...adent.org.uk> wrote:

>On Fri, 2012-09-28 at 11:51 -0700, Greg Kroah-Hartman wrote:
>> From: Greg KH <gregkh@...uxfoundation.org>
>> 
>> 3.5-stable review patch.  If anyone has any objections, please let me
>know.
>> 
>> ------------------
>> 
>> From: Suresh Siddha <suresh.b.siddha@...el.com>
>> 
>> commit c6fd893da927c6cefb2ece22402765379921a834 upstream.
>> 
>> Clear AVX, AVX2 features along with clearing XSAVE feature bits,
>> as part of the parsing "noxsave" parameter.
>> 
>> Fixes the kernel boot panic with "noxsave" boot parameter.
>> 
>> We could have checked cpu_has_osxsave along with cpu_has_avx etc, but
>Peter
>> mentioned clearing the feature bits will be better for uses like
>> static_cpu_has() etc.
>
>This was marked as needed for 3.5 only - presumably because the kernel
>wasn't using AVX before this - but don't we also need to clear these
>bits even in earlier versions so userland knows not to use it?
>
>Ben.
>
>> Signed-off-by: Suresh Siddha <suresh.b.siddha@...el.com>
>> Link:
>http://lkml.kernel.org/r/1343755754.2041.2.camel@sbsiddha-desk.sc.intel.com
>> Signed-off-by: H. Peter Anvin <hpa@...or.com>
>> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>> 
>> ---
>>  arch/x86/kernel/cpu/common.c |    2 ++
>>  1 file changed, 2 insertions(+)
>> 
>> --- a/arch/x86/kernel/cpu/common.c
>> +++ b/arch/x86/kernel/cpu/common.c
>> @@ -144,6 +144,8 @@ static int __init x86_xsave_setup(char *
>>  {
>>  	setup_clear_cpu_cap(X86_FEATURE_XSAVE);
>>  	setup_clear_cpu_cap(X86_FEATURE_XSAVEOPT);
>> +	setup_clear_cpu_cap(X86_FEATURE_AVX);
>> +	setup_clear_cpu_cap(X86_FEATURE_AVX2);
>>  	return 1;
>>  }
>>  __setup("noxsave", x86_xsave_setup);

-- 
Sent from my mobile phone. Please excuse brevity and lack of formatting.
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