linux-kernel - Re: [PATCH v2 08/39] x86/mm: Remove _PAGE

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Date:   Wed, 5 Oct 2022 01:31:28 +0000
From:   Andrew Cooper <Andrew.Cooper3@...rix.com>
To:     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>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>
CC:     Yu-cheng Yu <yu-cheng.yu@...el.com>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2 08/39] x86/mm: Remove _PAGE_DIRTY from kernel RO pages

On 29/09/2022 23:29, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@...el.com>
>
> Processors sometimes directly create Write=0,Dirty=1 PTEs.

Do they? (Rhetorical)

Yes, this is a relevant anecdote for why CET isn't available on pre-TGL
parts, but it one of the more wrong things to have as the first sentence
of this commit message.

The point you want to express is that under the CET-SS spec, R/O+Dirty
has a new meaning as type=shstk, so stop using this bit combination for
existing mappings.

I'm not even sure it's relevant to note that CET capable processors can
set D on a R/O mapping, because that depends on !CR0.WP which in turn
prohibits CR4.CET being enabled.

~Andrew