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

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Date:   Wed, 5 Oct 2022 23:06:53 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "bsingharora@...il.com" <bsingharora@...il.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "Syromiatnikov, Eugene" <esyr@...hat.com>,
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        "rdunlap@...radead.org" <rdunlap@...radead.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Eranian, Stephane" <eranian@...gle.com>,
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CC:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v2 10/39] x86/mm: Introduce _PAGE_COW

On Wed, 2022-10-05 at 07:08 -0700, Dave Hansen wrote:
> 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.

Although this text was inherited, I thought it was useful to disperse
any "huh, I wonder why" thoughts that may be lingering in the readers
head as they try to grok the rest of the text. I'll shorten it as
suggested. Thanks all.