From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dave Hansen <dave.hansen@linux.intel.com>
Subject: Re: [RFC PATCH v2 11/27] x86/mm: Modify ptep_set_wrprotect and
 pmdp_set_wrprotect for _PAGE_DIRTY_SW
Date: Tue, 10 Jul 2018 16:52:37 -0700
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To: Nadav Amit <nadav.amit@gmail.com>, Yu-cheng Yu <yu-cheng.yu@intel.com>, x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-api@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>, Andy Lutomirski <luto@amacapital.net>, Balbir Singh <bsingharora@gmail.com>, Cyrill Gorcunov <gorcunov@gmail.com>, Florian Weimer <fweimer@redhat.com>, "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>, Kees Cook <keescook@chromiun.org>, Mike Kravetz <mike.kravetz@oracle.com>, Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>, Peter Zijlstra <pet>
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On 07/10/2018 04:23 PM, Nadav Amit wrote:
> at 6:44 PM, Dave Hansen <dave.hansen@linux.intel.com> 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
>> 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.
> 
> Interesting. Does that regard the knights landing bug or something more
> general?

It's more general.

> Will the write succeed or trigger a page-fault in this case?

It will trigger a page fault.