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

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Date:   Thu, 21 Jan 2021 12:20:04 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>,
        Borislav Petkov <bp@...en8.de>
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,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...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>,
        Randy Dunlap <rdunlap@...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 12:16 PM, Yu, Yu-cheng 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.

Are you sure?

Usually, the compiler is better at making code efficient than humans.  I
find that coding it in the most human-readable way is best unless I
*know* the compiler is unable to generate god code.