linux-kernel - Re: [PATCH v2 10/39] x86/mm: Introduce _PAGE

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Date:   Wed, 5 Oct 2022 07:08:15 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.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>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        "joao.moreira@...el.com" <joao.moreira@...el.com>,
        John Allen <john.allen@....com>,
        "kcc@...gle.com" <kcc@...gle.com>,
        "eranian@...gle.com" <eranian@...gle.com>,
        "rppt@...nel.org" <rppt@...nel.org>,
        "jamorris@...ux.microsoft.com" <jamorris@...ux.microsoft.com>,
        "dethoma@...rosoft.com" <dethoma@...rosoft.com>
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v2 10/39] x86/mm: Introduce _PAGE_COW

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@...el.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.