From: "Andy Lutomirski" <luto@kernel.org>
To: "Mark Rutland" <mark.rutland@arm.com>
Cc: "Russell King (Oracle)" <linux@armlinux.org.uk>,
"the arch/x86 maintainers" <x86@kernel.org>,
"Dave Hansen" <dave.hansen@intel.com>,
"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
linux-mm@kvack.org, "Andrew Morton" <akpm@linux-foundation.org>,
"Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>,
"Nicholas Piggin" <npiggin@gmail.com>,
"Peter Zijlstra (Intel)" <peterz@infradead.org>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 7/8] membarrier: Remove arm (32) support for SYNC_CORE
Date: Thu, 17 Jun 2021 07:00:26 -0700 [thread overview]
Message-ID: <33241b25-4d45-4278-a4e6-ec9c12b0e1f3@www.fastmail.com> (raw)
In-Reply-To: <20210617135133.GA86101@C02TD0UTHF1T.local>
On Thu, Jun 17, 2021, at 6:51 AM, Mark Rutland wrote:
> On Thu, Jun 17, 2021 at 06:41:41AM -0700, Andy Lutomirski wrote:
> > In any event, I’m even more convinced that no new SYNC_CORE arches
> > should be added. We need a new API that just does the right thing.
>
> My intuition is the other way around, and that this is a gnereally
> useful thing for architectures that require context synchronization.
Except that you can't use it in a generic way. You have to know the specific rules for your arch.
>
> It's not clear to me what "the right thing" would mean specifically, and
> on architectures with userspace cache maintenance JITs can usually do
> the most optimal maintenance, and only need help for the context
> synchronization.
>
This I simply don't believe -- I doubt that any sane architecture really works like this. I wrote an email about it to Intel that apparently generated internal discussion but no results. Consider:
mmap(some shared library, some previously unmapped address);
this does no heavyweight synchronization, at least on x86. There is no "serializing" instruction in the fast path, and it *works* despite anything the SDM may or may not say.
We can and, IMO, should develop a sane way for user programs to install instructions into VMAs, for security-conscious software to verify them (by splitting the read and write sides?), and for their consumers to execute them, without knowing any arch details. And I think this can be done with no IPIs except for possible TLB flushing when needed, at least on most architectures. It would require a nontrivial amount of design work, and it would not resemble sys_cacheflush() or SYNC_CORE.
--Andy
next prev parent reply other threads:[~2021-06-17 14:00 UTC|newest]
Thread overview: 91+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-16 3:21 [PATCH 0/8] membarrier cleanups Andy Lutomirski
2021-06-16 3:21 ` [PATCH 1/8] membarrier: Document why membarrier() works Andy Lutomirski
2021-06-16 4:00 ` Nicholas Piggin
2021-06-16 7:30 ` Peter Zijlstra
2021-06-17 23:45 ` Andy Lutomirski
2021-06-16 3:21 ` [PATCH 2/8] x86/mm: Handle unlazying membarrier core sync in the arch code Andy Lutomirski
2021-06-16 4:25 ` Nicholas Piggin
2021-06-16 18:31 ` Andy Lutomirski
2021-06-16 17:49 ` Mathieu Desnoyers
2021-06-16 18:31 ` Andy Lutomirski
2021-06-16 3:21 ` [PATCH 3/8] membarrier: Remove membarrier_arch_switch_mm() prototype in core code Andy Lutomirski
2021-06-16 4:26 ` Nicholas Piggin
2021-06-16 17:52 ` Mathieu Desnoyers
2021-06-16 3:21 ` [PATCH 4/8] membarrier: Make the post-switch-mm barrier explicit Andy Lutomirski
2021-06-16 4:19 ` Nicholas Piggin
2021-06-16 7:35 ` Peter Zijlstra
2021-06-16 18:41 ` Andy Lutomirski
2021-06-17 1:37 ` Nicholas Piggin
2021-06-17 2:57 ` Andy Lutomirski
2021-06-17 5:32 ` Andy Lutomirski
2021-06-17 6:51 ` Nicholas Piggin
2021-06-17 23:49 ` Andy Lutomirski
2021-06-19 2:53 ` Nicholas Piggin
2021-06-19 3:20 ` Andy Lutomirski
2021-06-19 4:27 ` Nicholas Piggin
2021-06-17 9:08 ` [RFC][PATCH] sched: Use lightweight hazard pointers to grab lazy mms Peter Zijlstra
2021-06-17 9:10 ` Peter Zijlstra
2021-06-17 10:00 ` Nicholas Piggin
2021-06-17 9:13 ` Peter Zijlstra
2021-06-17 14:06 ` Andy Lutomirski
2021-06-17 9:28 ` Peter Zijlstra
2021-06-17 14:03 ` Andy Lutomirski
2021-06-17 14:10 ` Andy Lutomirski
2021-06-17 15:45 ` Peter Zijlstra
2021-06-18 3:29 ` Paul E. McKenney
2021-06-18 5:04 ` Andy Lutomirski
2021-06-17 15:02 ` [PATCH 4/8] membarrier: Make the post-switch-mm barrier explicit Paul E. McKenney
2021-06-18 0:06 ` Andy Lutomirski
2021-06-18 3:35 ` Paul E. McKenney
2021-06-17 8:45 ` Peter Zijlstra
2021-06-16 3:21 ` [PATCH 5/8] membarrier, kthread: Use _ONCE accessors for task->mm Andy Lutomirski
2021-06-16 4:28 ` Nicholas Piggin
2021-06-16 18:08 ` Mathieu Desnoyers
2021-06-16 18:45 ` Andy Lutomirski
2021-06-16 3:21 ` [PATCH 6/8] powerpc/membarrier: Remove special barrier on mm switch Andy Lutomirski
2021-06-16 4:36 ` Nicholas Piggin
2021-06-16 3:21 ` [PATCH 7/8] membarrier: Remove arm (32) support for SYNC_CORE Andy Lutomirski
2021-06-16 9:28 ` Russell King (Oracle)
2021-06-16 10:16 ` Peter Zijlstra
2021-06-16 10:20 ` Peter Zijlstra
2021-06-16 10:34 ` Russell King (Oracle)
2021-06-16 11:10 ` Peter Zijlstra
2021-06-16 13:22 ` Russell King (Oracle)
2021-06-16 15:04 ` Catalin Marinas
2021-06-16 15:23 ` Russell King (Oracle)
2021-06-16 15:45 ` Catalin Marinas
2021-06-16 16:00 ` Catalin Marinas
2021-06-16 16:27 ` Russell King (Oracle)
2021-06-17 8:55 ` Krzysztof Hałasa
2021-06-18 12:54 ` Linus Walleij
2021-06-18 13:19 ` Russell King (Oracle)
2021-06-18 13:36 ` Arnd Bergmann
2021-06-17 10:40 ` Mark Rutland
2021-06-17 11:23 ` Russell King (Oracle)
2021-06-17 11:33 ` Mark Rutland
2021-06-17 13:41 ` Andy Lutomirski
2021-06-17 13:51 ` Mark Rutland
2021-06-17 14:00 ` Andy Lutomirski [this message]
2021-06-17 14:20 ` Mark Rutland
2021-06-17 15:01 ` Peter Zijlstra
2021-06-17 15:13 ` Peter Zijlstra
2021-06-17 14:16 ` Mathieu Desnoyers
2021-06-17 14:05 ` Peter Zijlstra
2021-06-18 0:07 ` Andy Lutomirski
2021-06-16 3:21 ` [PATCH 8/8] membarrier: Rewrite sync_core_before_usermode() and improve documentation Andy Lutomirski
2021-06-16 4:45 ` Nicholas Piggin
2021-06-16 18:52 ` Andy Lutomirski
2021-06-16 23:48 ` Andy Lutomirski
2021-06-18 15:27 ` Christophe Leroy
2021-06-16 10:20 ` Will Deacon
2021-06-16 23:58 ` Andy Lutomirski
2021-06-17 14:47 ` Mathieu Desnoyers
2021-06-18 0:12 ` Andy Lutomirski
2021-06-18 16:31 ` Mathieu Desnoyers
2021-06-18 19:58 ` Andy Lutomirski
2021-06-18 20:09 ` Mathieu Desnoyers
2021-06-19 6:02 ` Nicholas Piggin
2021-06-19 15:50 ` Andy Lutomirski
2021-06-20 2:10 ` Nicholas Piggin
2021-06-17 15:16 ` Mathieu Desnoyers
2021-06-18 0:13 ` Andy Lutomirski
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