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

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Message-ID: <Yz1ncw+up1nRb9Eo@hirez.programming.kicks-ass.net>
Date:   Wed, 5 Oct 2022 13:16:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Cooper <Andrew.Cooper3@...rix.com>
Cc:     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>,
        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>,
        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 Wed, Oct 05, 2022 at 01:31:28AM +0000, Andrew Cooper wrote:
> 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.

Whilst I agree that the Changelog is 'suboptimal' -- I do think it might
be good to mention how we ended up at the current state where we
explicitly set this non-sensical W=0,D=1 state.

Looking at the git history this seems to be a bit of a hysterical
accident, not something done on purpose to 'optimize' for these weird
CPUs.