linux-kernel - Re: [PATCH v4 07/10] x86/xsaves: Fix PTRACE frames for XSAVES

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Date:	Fri, 29 Apr 2016 15:36:12 -0700
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 v4 07/10] x86/xsaves: Fix PTRACE frames for XSAVES

On 04/29/2016 03:30 PM, Yu-cheng Yu wrote:
> On Fri, Apr 29, 2016 at 01:25:34PM -0700, Dave Hansen wrote:
>> On 03/04/2016 10:12 AM, Yu-cheng Yu wrote:
>>> +	for (i = 0; i < XFEATURE_MAX; i++) {
>>> +		/*
>>> +		 * Copy only in-use xstates.
>>> +		 */
>>> +		if (((header.xfeatures >> i) & 1) && xfeature_enabled(i)) {
>>> +			void *src = get_xsave_addr_no_check(xsave, i);
>>
>> How could a bit in header.xfeatures get set if it is not set in
>> xfeature_enabled() aka xfeatures_mask aka XCR0?
> 
> Do you mean, we should test xfeature_enabled(i) first, like,
> 
> 	if (xfeature_enabled(i) && ((header.xfeatures >> i) & 1)) ?
> 
> The result will be the same, like you said, if XCR0[i] is not set,
> hader.xfeatures[i] cannot be set.  But if XCR0[i] is set,
> header.xfeatures[i] can be cleared. 

I think the xfeature_enabled(i) is probably redundant.  Does it serve
any actual purpose?