From: Andy Lutomirski <luto@kernel.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andy Lutomirski <luto@kernel.org>, paulmck <paulmck@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
"Russell King, ARM Linux" <linux@armlinux.org.uk>,
x86 <x86@kernel.org>, linux-kernel <linux-kernel@vger.kernel.org>,
Nicholas Piggin <npiggin@gmail.com>,
Arnd Bergmann <arnd@arndb.de>,
Michael Ellerman <mpe@ellerman.id.au>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
stable <stable@vger.kernel.org>
Subject: Re: [RFC please help] membarrier: Rewrite sync_core_before_usermode()
Date: Mon, 28 Dec 2020 16:30:39 -0800 [thread overview]
Message-ID: <CALCETrWaYU26z6RzHCN+VsTvS-uhApjr+jahS85de6WB_V37Tg@mail.gmail.com> (raw)
In-Reply-To: <1670059472.3671.1609189779376.JavaMail.zimbra@efficios.com>
On Mon, Dec 28, 2020 at 1:09 PM Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
>
> ----- On Dec 27, 2020, at 4:36 PM, Andy Lutomirski luto@kernel.org wrote:
>
> [...]
>
> >> You seem to have noticed odd cases on arm64 where this guarantee does not
> >> match reality. Where exactly can we find this in the code, and which part
> >> of the architecture manual can you point us to which supports your concern ?
> >>
> >> Based on the notes I have, use of `eret` on aarch64 guarantees a context
> >> synchronizing
> >> instruction when returning to user-space.
> >
> > Based on my reading of the manual, ERET on ARM doesn't synchronize
> > anything at all. I can't find any evidence that it synchronizes data
> > or instructions, and I've seen reports that the CPU will happily
> > speculate right past it.
>
> Reading [1] there appears to be 3 kind of context synchronization events:
>
> - Taking an exception,
> - Returning from an exception,
> - ISB.
My reading of [1] is that all three of these are "context
synchronization event[s]", but that only ISB flushes the pipeline,
etc. The little description of context synchronization seems to
suggest that it only implies that certain register changes become
effective.
>
> This other source [2] adds (search for Context synchronization operation):
>
> - Exit from Debug state
> - Executing a DCPS instruction
> - Executing a DRPS instruction
>
> "ERET" falls into the second kind of events, and AFAIU should be context
> synchronizing. That was confirmed to me by Will Deacon when membarrier
> sync-core was implemented for aarch64. If the architecture reference manuals
> are wrong, is there an errata ?
>
> As for the algorithm to use on ARMv8 to update instructions, see [2]
> B2.3.4 Implication of caches for the application programmer
> "Synchronization and coherency issues between data and instruction accesses"
This specifically discusses ISB.
Let's wait for an actual ARM64 expert to chime in, though.
next prev parent reply other threads:[~2020-12-29 0:31 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 [this message]
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
2021-01-05 22:41 ` Will Deacon
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