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:   Fri, 31 Aug 2018 10:52:02 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Jann Horn <jannh@...gle.com>, Yu-cheng Yu <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>,
        "H. J. Lu" <hjl.tools@...il.com>, Jonathan Corbet <corbet@....net>,
        Kees Cook <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" <ravi.v.shankar@...el.com>,
        "Shanbhogue, Vedvyas" <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 08/31/2018 10:46 AM, Andy Lutomirski wrote:
> On Thu, Aug 30, 2018 at 11:55 AM, Dave Hansen
>> 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.
> 
> Can we get something much stronger, perhaps?  Like this:
> 
> On processors enumerating support for CET, the processor will write to
> the accessed and/or dirty flags atomically, as if using the LOCK
> CMPXCHG instruction.  The memory access, any cached entries in any
> paging-structure caches, and the values in the paging-structure entry
> before and after writing the A and/or D bits will all be consistent.

There's some talk of this already in: 8.1.2.1 Automatic Locking:

> When updating page-directory and page-table entries — When updating 
> page-directory and page-table entries, the processor uses locked 
> cycles to set the accessed and dirty flag in the page-directory and
> page-table entries.
As for the A/D consistency, I'll see if I can share that before it hits
the SDM for real and see if it's sufficient for everybody.