linux-kernel - Re: [PATCH v5 02/13] x86/xsaves: Rename xstate_size to kernel_xstate_size to explicitly distinguish xstate size in kernel from user space

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Date:	Tue, 10 May 2016 10:08:44 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Borislav Petkov <bp@...e.de>, Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	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 v5 02/13] x86/xsaves: Rename xstate_size to
 kernel_xstate_size to explicitly distinguish xstate size in kernel from user
 space

On 05/10/2016 10:01 AM, Borislav Petkov wrote:
>> >  	pr_info("x86/fpu: Enabled xstate features 0x%llx, context size is %d bytes, using '%s' format.\n",
>> >  		xfeatures_mask,
>> > -		xstate_size,
>> > +		kernel_xstate_size,
>> >  		cpu_has_xsaves ? "compacted" : "standard");
> I think we should dump user_xstate_size in the compacted case since it
> is != kernel_xstate_size.

Why?  "kernel_xstate_size" is important to the kernel because it impacts
task_struct size.

But the kernel never actually stores "user_xstate_size" anywhere or
really ever even cares about it except when copying in/out of userspace.
 "user_xstate_size" is also entirely enumerable in userspace with a
single cpuid instruction.

It's nice to dump out interesting data in dmesg, but I'm curious why you
think it's interesting.