From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Will Deacon <will@kernel.org>, Andy Lutomirski <luto@kernel.org>,
Nicholas Piggin <npiggin@gmail.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
X86 ML <x86@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Catalin Marinas <catalin.marinas@arm.com>,
linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
LKML <linux-kernel@vger.kernel.org>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
Michael Ellerman <mpe@ellerman.id.au>,
Paul Mackerras <paulus@samba.org>,
stable <stable@vger.kernel.org>
Subject: Re: [RFC please help] membarrier: Rewrite sync_core_before_usermode()
Date: Tue, 5 Jan 2021 17:37:31 +0100 [thread overview]
Message-ID: <20210105163731.GM3040@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <7BFAB97C-1949-46A3-A1E2-DFE108DC7D5E@amacapital.net>
On Tue, Jan 05, 2021 at 08:20:51AM -0800, Andy Lutomirski wrote:
> > Interestingly, the architecture recently added a control bit to remove
> > this synchronisation from exception return, so if we set that then we'd
> > have a problem with SYNC_CORE and adding an ISB would be necessary (and
> > we could probable then make kernel->kernel returns cheaper, but I
> > suspect we're relying on this implicit synchronisation in other places
> > too).
> >
>
> Is ISB just a context synchronization event or does it do more?
IIRC it just the instruction sync (like power ISYNC).
> On x86, it’s very hard to tell that MFENCE does any more than LOCK,
> but it’s much slower. And we have LFENCE, which, as documented,
> doesn’t appear to have any semantics at all. (Or at least it didn’t
> before Spectre.)
AFAIU MFENCE is a completion barrier, while LOCK prefix is not. A bit
like ARM's DSB vs DMB.
It is for this reason that mb() is still MFENCE, while our smp_mb() is a
LOCK prefixed NO-OP.
And yes, LFENCE used to be poorly defined and it was sometimes
understood to be a completion barrier relative to prior LOADs, while it
is now a completion barrier for any prior instruction, and really should
be renamed to IFENCE.
next prev parent reply other threads:[~2021-01-05 16:39 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-27 18:28 [RFC please help] membarrier: Rewrite sync_core_before_usermode() Andy Lutomirski
2020-12-27 20:18 ` Mathieu Desnoyers
2020-12-27 21:36 ` Andy Lutomirski
2020-12-28 10:25 ` Russell King - ARM Linux admin
2020-12-28 17:14 ` Andy Lutomirski
2020-12-28 17:23 ` Russell King - ARM Linux admin
2020-12-28 18:10 ` Andy Lutomirski
2020-12-28 18:29 ` Jann Horn
2020-12-28 18:50 ` Andy Lutomirski
2020-12-28 19:08 ` Russell King - ARM Linux admin
2020-12-28 19:44 ` Andy Lutomirski
2020-12-28 20:24 ` Russell King - ARM Linux admin
2020-12-28 20:40 ` Mathieu Desnoyers
2020-12-28 20:32 ` Mathieu Desnoyers
2020-12-28 21:06 ` Andy Lutomirski
2020-12-28 21:26 ` Mathieu Desnoyers
2020-12-29 0:36 ` Nicholas Piggin
2020-12-29 0:56 ` Andy Lutomirski
2020-12-29 3:09 ` Nicholas Piggin
2020-12-29 10:44 ` Russell King - ARM Linux admin
2020-12-30 2:33 ` Nicholas Piggin
2020-12-30 10:00 ` Russell King - ARM Linux admin
2020-12-30 10:58 ` Russell King - ARM Linux admin
2020-12-30 11:57 ` Nicholas Piggin
2020-12-28 21:09 ` Mathieu Desnoyers
2020-12-29 0:30 ` Andy Lutomirski
2020-12-29 0:11 ` Nicholas Piggin
2020-12-29 0:36 ` Andy Lutomirski
2020-12-29 3:31 ` Nicholas Piggin
2021-01-01 18:33 ` David Laight
2021-01-05 13:26 ` Will Deacon
2021-01-05 16:20 ` Andy Lutomirski
2021-01-05 16:37 ` Peter Zijlstra [this message]
2021-01-05 22:41 ` Will Deacon
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