linux-kernel - Re: [PATCH] Fix: xtensa: add missing sync

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Date:   Mon, 11 Sep 2017 21:37:33 -0700
From:   Max Filippov <jcmvbkbc@...il.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Chris Zankel <chris@...kel.net>,
        "linux-xtensa@...ux-xtensa.org" <linux-xtensa@...ux-xtensa.org>
Subject: Re: [PATCH] Fix: xtensa: add missing sync_core

Hi Mathieu,

On Tue, Aug 29, 2017 at 11:55 AM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
> ----- On Aug 28, 2017, at 1:12 PM, Max Filippov jcmvbkbc@...il.com wrote:
>> On Mon, Aug 28, 2017 at 12:36 AM, Mathieu Desnoyers
>> <mathieu.desnoyers@...icios.com> wrote:
>>> The membarrier system call now requires all architectures to implement
>>> sync_core(). On Xtensa, it is provided by the EXTW instruction.
>>>
>>> [ Completely untested! Can someone on the xtensa side confirm whether
>>>   EXTW is the right way to serialize core execution and try it out ? ]
>>
>> Thanks for the patch. I'm currently travelling, I'll give it a try next week
>> once I'm back at work.
>
> I think we may need to flush the icache to make it consistent with the dcache
> too on xtensa, in addition to the EXTW. The goal here is to allow JIT engines
> to reclaim and re-use memory after they discard dynamically generated code.
> This is similar to what we'd need to do on arm32, where they have inconsistent
> d/i-caches.

my understanding is that to support JIT engines on xtensa we need to do
icache/dcache synchronization, this procedure is covered in the ISYNC
instruction description in the ISA book, it involves MEMW and ISYNC,
but not EXTW. EXTW is meant to work as a CPU barrier that orders all
externally visible CPU signals, which seems unnecessary.

Interestingly, currently we don't have MEMW between dcache flush and
icache invalidation, so I need to add it to be consistent with the documented
procedure. Then I believe that sync_core implementation should invoke
flush_dcache_all followed by MEMW followed by invalidate_icache_all.
Does that sound right?

-- 
Thanks.
-- Max