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

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Date:   Thu, 21 Jan 2021 22:16:08 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'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>,
        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

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?
There is only an |= against the target.

Something horrid with ^= might set and clear.

	David

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