From: "Andy Lutomirski" <luto@kernel.org>
To: "Mark Rutland" <mark.rutland@arm.com>,
"Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: "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 06:41:41 -0700 [thread overview]
Message-ID: <394219d4-36a6-4e7f-a03c-8590551b099a@www.fastmail.com> (raw)
In-Reply-To: <20210617113349.GB82133@C02TD0UTHF1T.local>
On Thu, Jun 17, 2021, at 4:33 AM, Mark Rutland wrote:
> On Thu, Jun 17, 2021 at 12:23:05PM +0100, Russell King (Oracle) wrote:
> > On Thu, Jun 17, 2021 at 11:40:46AM +0100, Mark Rutland wrote:
> > > On Tue, Jun 15, 2021 at 08:21:12PM -0700, Andy Lutomirski wrote:
> > > > On arm32, the only way to safely flush icache from usermode is to call
> > > > cacheflush(2). This also handles any required pipeline flushes, so
> > > > membarrier's SYNC_CORE feature is useless on arm. Remove it.
> > >
> > > Unfortunately, it's a bit more complicated than that, and these days
> > > SYNC_CORE is equally necessary on arm as on arm64. This is something
> > > that changed in the architecture over time, but since ARMv7 we generally
> > > need both the cache maintenance *and* a context synchronization event
> > > (the latter must occur on the CPU which will execute the instructions).
> > >
> > > If you look at the latest ARMv7-AR manual (ARM DDI 406C.d), section
> > > A3.5.4 "Concurrent modification and execution of instructions" covers
> > > this. That manual can be found at:
> > >
> > > https://developer.arm.com/documentation/ddi0406/latest/
> >
> > Looking at that, sys_cacheflush() meets this. The manual details a
> > series of cache maintenance calls in "step 1" that the modifying thread
> > must issue - this is exactly what sys_cacheflush() does. The same is
> > true for ARMv6, except the "ISB" terminology is replaced by a
> > "PrefetchFlush" terminology. (I checked DDI0100I).
> >
> > "step 2" requires an ISB on the "other CPU" prior to executing that
> > code. As I understand it, in ARMv7, userspace can issue an ISB itself.
> >
> > For ARMv6K, it doesn't have ISB, but instead has a CP15 instruction
> > for this that isn't availble to userspace. This is where we come to
> > the situation about ARM 11MPCore, and whether we continue to support
> > it or not.
> >
> > So, I think we're completely fine with ARMv7 under 32-bit ARM kernels
> > as userspace has everything that's required. ARMv6K is a different
> > matter as we've already identified for several reasons.
>
> Sure, and I agree we should not change cacheflush().
>
> The point of membarrier(SYNC_CORE) is that you can move the cost of that
> ISB out of the fast-path in the executing thread(s) and into the
> slow-path on the thread which generated the code.
>
> So e.g. rather than an executing thread always having to do:
>
> LDR <reg>, [<funcptr>]
> ISB // in case funcptr was just updated
> BLR <reg>
>
> ... you have the thread generating the code use membarrier(SYNC_CORE)
> prior to plublishing the funcptr, and the fast-path on all the executing
> threads can be:
>
> LDR <reg> [<funcptr>]
> BLR <reg>
>
> ... and thus I think we still want membarrier(SYNC_CORE) so that people
> can do this, even if there are other means to achieve the same
> functionality.
I had the impression that sys_cacheflush() did that. Am I wrong?
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.
next prev parent reply other threads:[~2021-06-17 13:42 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 [this message]
2021-06-17 13:51 ` Mark Rutland
2021-06-17 14:00 ` Andy Lutomirski
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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