RE: [PATCH v17 08/26] x86/mm: Introduce _PAGE_COW

[David Laight]

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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