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

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Message-ID: <CAG48ez3hXfsUkMqcHmVetzywKC8a+PLhGReceTdwCf7B03Oj7g@mail.gmail.com>
Date:   Fri, 30 Sep 2022 17:16:12 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     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-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...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>,
        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, John Allen <john.allen@....com>,
        kcc@...gle.com, eranian@...gle.com, rppt@...nel.org,
        jamorris@...ux.microsoft.com, dethoma@...rosoft.com,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v2 10/39] x86/mm: Introduce _PAGE_COW

On Fri, Sep 30, 2022 at 12:30 AM Rick Edgecombe
<rick.p.edgecombe@...el.com> wrote:
> 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.

Stupid question, since I just recently learned that IOMMUv2 is a
thing: I assume this also holds for IOMMUs that implement IOMMUv2/SVA,
where the IOMMU directly walks the userspace page tables, and not just
for the CPU core?