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Date:   Thu, 10 Dec 2020 18:41:55 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Cc:     x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
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Subject: Re: [PATCH v15 08/26] x86/mm: Introduce _PAGE_COW
Message-ID: <20201210174155.GD26529@zn.tnic>
References: <20201110162211.9207-1-yu-cheng.yu@intel.com>
 <20201110162211.9207-9-yu-cheng.yu@intel.com>
 <20201208175014.GD27920@zn.tnic>
 <218503f6-eec1-94b0-8404-6f92c55799e3@intel.com>
 <20201208184727.GF27920@zn.tnic>
 <cddc2cc5-a04e-ce9c-6fdf-2e7a29346cf7@intel.com>
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On Tue, Dec 08, 2020 at 11:24:16AM -0800, Yu, Yu-cheng wrote:
> Case (a) is a normal writable data page that has gone through fork(). So it

Writable?

> has W=0, D=1.  But here, the software chooses not to use the D bit, and

But it has W=0. So not writable?

> instead, W=0, COW=1.

So the "new" way of denoting that the page is modified is COW=1
*when* on CET hw. The D=1 bit is still used on the rest thus the two
_PAGE_DIRTY_BITS.

Am I close?

> Case (b) is a normal read-only data page.  Since it is read-only, fork()
> won't affect it.  In __get_user_pages(), a copy of the read-only page is
> needed, and the page is duplicated.  The software sets COW=1 for the new
> copy.

That makes more sense.

> Thread-A is writing to a writable page, and the page's PTE is becoming W=1,
> D=1.  In the middle of it, Thread-B is changing the PTE to W=0.

Yah, add that to the explanation pls.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette