linux-kernel - Re: [PATCH v2 2/5] mm: avoid unnecessary flush on change_huge

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Message-ID: <38e5196e-4ea1-2d87-007f-0ff69e7e1063@intel.com>
Date:   Tue, 26 Oct 2021 13:47:38 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Nick Piggin <npiggin@...il.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v2 2/5] mm: avoid unnecessary flush on change_huge_pmd()

On 10/26/21 1:07 PM, Nadav Amit wrote:
> I just wonder how come the R/W-clearing and the P-clearing cause concurrent
> dirty bit setting to behave differently. I am not a hardware guy, but I would
> imagine they would be the same...

First of all, I think the non-atomic properties where a PTE can go:

	W=1,D=0 // original
	W=0,D=0 // software clears W
	W=0,D=1 // hardware sets D

were a total implementation accident.  It wasn't someone being clever
and since the behavior was architecturally allowed and well-tolerated by
software it was around for a while.  I think I was the one that asked
that it get fixed for shadow stacks, and nobody pushed back on it too
hard as far as I remember.  I don't think it was super hard to fix.

Why do the Present/Accessed and Write/Dirty pairs act differently?  I
think it's a total implementation accident and wasn't by design.

The KNL erratum was an erratum and wasn't codified in the architecture
because it actually broke things.  The pre-CET Write/Dirty behavior
didn't break software  to a level it was considered an erratum.  It gets
to live on as allowed in the architecture.