linux-kernel - Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY

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Date:   Thu, 30 Aug 2018 11:55:14 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Jann Horn <jannh@...gle.com>, yu-cheng.yu@...el.com,
        the arch/x86 maintainers <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Florian Weimer <fweimer@...hat.com>, hjl.tools@...il.com,
        Jonathan Corbet <corbet@....net>, keescook@...omiun.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>,
        ravi.v.shankar@...el.com, vedvyas.shanbhogue@...el.com
Subject: Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and
 pmdp_set_wrprotect for _PAGE_DIRTY_SW

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.