linux-kernel - Re: [PATCH v17 08/26] x86/mm: Introduce _PAGE

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Date:   Thu, 21 Jan 2021 14:19:28 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     David Laight <David.Laight@...LAB.COM>,
        "'Yu, Yu-cheng'" <yu-cheng.yu@...el.com>,
        Borislav Petkov <bp@...en8.de>
Cc:     "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>
Subject: Re: [PATCH v17 08/26] x86/mm: Introduce _PAGE_COW

On 1/21/21 2:16 PM, David Laight wrote:
> From: Yu, Yu-cheng 
>>
>> On 1/21/2021 10:44 AM, Borislav Petkov wrote:
>>> On Tue, Dec 29, 2020 at 01:30:35PM -0800, Yu-cheng Yu wrote:
>> [...]
>>>> @@ -343,6 +349,16 @@ static inline pte_t pte_mkold(pte_t pte)
>>>>
>>>>   static inline pte_t pte_wrprotect(pte_t pte)
>>>>   {
>>>> +	/*
>>>> +	 * Blindly clearing _PAGE_RW might accidentally create
>>>> +	 * a shadow stack PTE (RW=0, Dirty=1).  Move the hardware
>>>> +	 * dirty value to the software bit.
>>>> +	 */
>>>> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
>>>> +		pte.pte |= (pte.pte & _PAGE_DIRTY) >> _PAGE_BIT_DIRTY << _PAGE_BIT_COW;
>>>
>>> Why the unreadable shifting when you can simply do:
>>>
>>>                  if (pte.pte & _PAGE_DIRTY)
>>>                          pte.pte |= _PAGE_COW;
>>>
> 
>>> ?
>>
>> It clears _PAGE_DIRTY and sets _PAGE_COW.  That is,
>>
>> if (pte.pte & _PAGE_DIRTY) {
>> 	pte.pte &= ~_PAGE_DIRTY;
>> 	pte.pte |= _PAGE_COW;
>> }
>>
>> So, shifting makes resulting code more efficient.
> 
> Does the compiler manage to do one shift?
> 
> How can it clear anything?

It could shift it off either end since there are both
<< and >>.

> There is only an |= against the target.
> 
> Something horrid with ^= might set and clear.


-- 
~Randy
"He closes his eyes and drops the goggles.  You can't get hurt
by looking at a bitmap.  Or can you?"
(Neal Stephenson: Snow Crash)