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Message-ID: <715095e6-6c4e-62dd-6631-b096db2cd92c@intel.com>
Date:   Wed, 5 Oct 2022 07:08:15 -0700
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Subject: Re: [PATCH v2 10/39] x86/mm: Introduce _PAGE_COW
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To:     Andrew Cooper <Andrew.Cooper3@citrix.com>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        "x86@kernel.org" <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-kernel@vger.kernel.org>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
        "linux-api@vger.kernel.org" <linux-api@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.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@chromium.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@intel.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        "joao.moreira@intel.com" <joao.moreira@intel.com>,
        John Allen <john.allen@amd.com>,
        "kcc@google.com" <kcc@google.com>,
        "eranian@google.com" <eranian@google.com>,
        "rppt@kernel.org" <rppt@kernel.org>,
        "jamorris@linux.microsoft.com" <jamorris@linux.microsoft.com>,
        "dethoma@microsoft.com" <dethoma@microsoft.com>
Cc:     Yu-cheng Yu <yu-cheng.yu@intel.com>
References: <20220929222936.14584-1-rick.p.edgecombe@intel.com>
 <20220929222936.14584-11-rick.p.edgecombe@intel.com>
 <54cdad9f-b810-7966-5928-9320d970a43d@citrix.com>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <54cdad9f-b810-7966-5928-9320d970a43d@citrix.com>
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On 10/4/22 19:17, Andrew Cooper wrote:
> On 29/09/2022 23:29, Rick Edgecombe wrote:
>> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
>>
>> There is essentially no room left in the x86 hardware PTEs on some OSes
>> (not Linux). That left the hardware architects looking for a way to
>> represent a new memory type (shadow stack) within the existing bits.
>> They chose to repurpose a lightly-used state: Write=0,Dirty=1.
> How does "Some OSes have a greater dependence on software available bits
> in PTEs than Linux" sound?
> 
>> The reason it's lightly used is that Dirty=1 is normally set _before_ a
>> write. A write with a Write=0 PTE would typically only generate a fault,
>> not set Dirty=1. Hardware can (rarely) both set Write=1 *and* generate the
>> fault, resulting in a Dirty=0,Write=1 PTE. Hardware which supports shadow
>> stacks will no longer exhibit this oddity.
> Again, an interesting anecdote but not salient information here.

As much as I like the sound of my own voice (and anecdotes), I agree
that this is a bit oblique for the patch.  Maybe this anecdote should
get banished elsewhere.

The changelog here could definitely get to the point faster.