Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY_SW

[Dave Hansen]

On 08/30/2018 10:34 AM, Andy Lutomirski wrote:
>> But, to keep B's TLB from picking up the entry, I think we can just make
>> it !Present for a moment.  No TLB can cache it, and I believe the same
>> "don't set Dirty on a !Writable entry" logic also holds for !Present
>> (modulo a weird erratum or two).
> Can we get documentation?  Pretty please?

The accessed bit description in the SDM looks pretty good to me today:

> Whenever the processor uses a paging-structure entry as part of
> linear-address translation, it sets the accessed flag in that entry
> (if it is not already set).
If it's !Present, it can't used as part of a translation so can't be
set.  I think that covers the thing I was unsure about.

But, Dirty is a bit, er, muddier, but mostly because it only gets set on
leaf entries:

> Whenever there is a write to a linear address, the processor sets the
> dirty flag (if it is not already set) in the paging- structure entry
> that identifies the final physical address for the linear address
> (either a PTE or a paging-structure entry in which the PS flag is
> 1).

That little hunk will definitely need to get updated with something like:

	On processors enumerating support for CET, the processor will on
	set the dirty flag on paging structure entries in which the W
	flag is 1.