From: Dave Hansen <dave.hansen@intel.com>
To: Nadav Amit <nadav.amit@gmail.com>
Cc: Linux-MM <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Andrew Cooper <andrew.cooper3@citrix.com>,
Andrew Morton <akpm@linux-foundation.org>,
Andy Lutomirski <luto@kernel.org>,
Dave Hansen <dave.hansen@linux.intel.com>,
Peter Xu <peterx@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
Nick Piggin <npiggin@gmail.com>,
"x86@kernel.org" <x86@kernel.org>
Subject: Re: [PATCH v2 2/5] mm: avoid unnecessary flush on change_huge_pmd()
Date: Tue, 26 Oct 2021 13:47:38 -0700 [thread overview]
Message-ID: <38e5196e-4ea1-2d87-007f-0ff69e7e1063@intel.com> (raw)
In-Reply-To: <640A6374-A06B-4E20-BF5D-9A21CC85CB12@gmail.com>
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.
next prev parent reply other threads:[~2021-10-26 20:47 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-21 12:21 [PATCH v2 0/5] mm/mprotect: avoid unnecessary TLB flushes Nadav Amit
2021-10-21 12:21 ` [PATCH v2 1/5] x86: Detection of Knights Landing A/D leak Nadav Amit
2021-10-26 15:54 ` Dave Hansen
2021-10-26 15:57 ` Nadav Amit
2021-10-21 12:21 ` [PATCH v2 2/5] mm: avoid unnecessary flush on change_huge_pmd() Nadav Amit
2021-10-25 10:52 ` Peter Zijlstra
2021-10-25 16:29 ` Nadav Amit
2021-10-26 16:06 ` Dave Hansen
2021-10-26 16:47 ` Nadav Amit
2021-10-26 16:53 ` Nadav Amit
2021-10-26 17:44 ` Nadav Amit
2021-10-26 18:44 ` Dave Hansen
2021-10-26 19:06 ` Nadav Amit
2021-10-26 19:40 ` Dave Hansen
2021-10-26 20:07 ` Nadav Amit
2021-10-26 20:47 ` Dave Hansen [this message]
2021-10-21 12:21 ` [PATCH v2 3/5] x86/mm: check exec permissions on fault Nadav Amit
2021-10-25 10:59 ` Peter Zijlstra
2021-10-25 11:13 ` Andrew Cooper
2021-10-25 14:23 ` Dave Hansen
2021-10-25 14:20 ` Dave Hansen
2021-10-25 16:19 ` Nadav Amit
2021-10-25 17:45 ` Dave Hansen
2021-10-25 17:51 ` Nadav Amit
2021-10-25 18:00 ` Dave Hansen
2021-10-21 12:21 ` [PATCH v2 4/5] mm/mprotect: use mmu_gather Nadav Amit
2021-10-21 12:21 ` [PATCH v2 5/5] mm/mprotect: do not flush on permission promotion Nadav Amit
2021-10-25 11:12 ` Peter Zijlstra
2021-10-25 16:27 ` Nadav Amit
2021-10-22 3:04 ` [PATCH v2 0/5] mm/mprotect: avoid unnecessary TLB flushes Andrew Morton
2021-10-22 21:58 ` Nadav Amit
2021-10-26 16:09 ` Dave Hansen
2021-10-25 10:50 ` Peter Zijlstra
2021-10-25 16:42 ` Nadav Amit
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