Re: [RFC PATCH v2 11/27] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY_SW

[Peter Zijlstra]

On Tue, Jul 10, 2018 at 03:44:32PM -0700, Dave Hansen wrote:
> On 07/10/2018 03:26 PM, Yu-cheng Yu wrote:
> > +	/*
> > +	 * On platforms before CET, other threads could race to
> > +	 * create a RO and _PAGE_DIRTY_HW PMD again.  However,
> > +	 * on CET platforms, this is safe without a TLB flush.
> > +	 */
> 
> If I didn't work for Intel, I'd wonder what the heck CET is and what the
> heck it has to do with _PAGE_DIRTY_HW.  I think we need a better comment

And Changelog, the provided one is abysmal.

> than this.  How about:
> 
> 	Some processors can _start_ a write, but end up seeing
> 	a read-only PTE by the time they get to getting the
> 	Dirty bit.  In this case, they will set the Dirty bit,
> 	leaving a read-only, Dirty PTE which looks like a Shadow
> 	Stack PTE.
> 
> 	However, this behavior has been improved and will *not* occur on
> 	processors supporting Shadow Stacks.  Without this guarantee, a
> 	transition to a non-present PTE and flush the TLB would be
> 	needed.

I'm still struggling. I think I get the first paragraph, but then what?