linux-kernel - Re: [PATCH v3 9/9] x86/xsaves: Re-enable XSAVES

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Date:	Tue, 1 Mar 2016 16:45:41 -0800
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:	x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...e.de>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [PATCH v3 9/9] x86/xsaves: Re-enable XSAVES

On 03/01/2016 04:34 PM, Yu-cheng Yu wrote:
> On Tue, Mar 01, 2016 at 03:56:12PM -0800, Dave Hansen wrote:
>> On 02/29/2016 09:42 AM, Yu-cheng Yu wrote:
>>> -	setup_clear_cpu_cap(X86_FEATURE_XSAVES);
>>> +	if (!config_enabled(CONFIG_X86_64))
>>> +		setup_clear_cpu_cap(X86_FEATURE_XSAVES);
>>>  }
>>
>> I think we need a much better explanation of this for posterity.  Why
>> are we not supporting this now, and what would someone have to do in the
>> future in order to enable it?
>>
> If anyone is using this newer feature, then that user is most likely using
> a 64-bit capable processor and a 64-bit kernel. The intention here is to
> take out the complexity and any potential of error. If the user removes 
> the restriction and builds a private kernel, it should work but we have
> not checked all possible combinations. I will put these in the comments.

A user can go download a 32-bit version of Ubuntu or Debian and install
it on a 64-bit processor today.  It's a very easy mistake to make when
downloading the install CD.

In any case, I don't have a _problem_ with leaving i386 in the dust
here.  I just want us to be very explicit about what we are doing.

>>> +	/*
>>> +	 * Make it clear that XSAVES supervisor states are not yet
>>> +	 * implemented should anyone expect it to work by changing
>>> +	 * bits in XFEATURE_MASK_* macros and XCR0.
>>> +	 */
>>> +	WARN_ONCE((xfeatures_mask & XFEATURE_MASK_SUPERVISOR),
>>> +		"x86/fpu: XSAVES supervisor states are not yet implemented.\n");
>>> +
>>>  	cr4_set_bits(X86_CR4_OSXSAVE);
>>>  	xsetbv(XCR_XFEATURE_ENABLED_MASK, xfeatures_mask);
>>>  }
>>
>> Let's also do a:
>>
>> 	xfeatures_mask &= ~XFEATURE_MASK_SUPERVISOR;
>>
>> Otherwise, we have a broken system at the moment.
>>
> Currently, if anyone sets any supervisor state in xfeatures_mask, the
> kernel prints out the warning then goes into a protection fault.
> That is a very strong indication to the user. Do we want to mute it? 

By "goes into a protection fault", do you mean that it doesn't boot?

I'd just rather we put the kernel in a known-safe configuration (masking
supervisor state out of xfeatures_mask) rather than rely on the general
protection fault continuing to be generated by whatever is generating it.