Re: [PATCH 7/8] membarrier: Remove arm (32) support for SYNC_CORE

From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Catalin Marinas <catalin.marinas@arm.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	Krzysztof Halasa <khalasa@piap.pl>,
	Neil Armstrong <narmstrong@baylibre.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>,
	x86@kernel.org, Dave Hansen <dave.hansen@intel.com>,
	LKML <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>,
	linux-arm-kernel@lists.infradead.org,
	Will Deacon <will@kernel.org>
Subject: Re: [PATCH 7/8] membarrier: Remove arm (32) support for SYNC_CORE
Date: Wed, 16 Jun 2021 17:27:16 +0100	[thread overview]
Message-ID: <20210616162716.GH22278@shell.armlinux.org.uk> (raw)
In-Reply-To: <20210616160050.GE22433@arm.com>

On Wed, Jun 16, 2021 at 05:00:51PM +0100, Catalin Marinas wrote:
> On Wed, Jun 16, 2021 at 04:45:29PM +0100, Catalin Marinas wrote:
> > On Wed, Jun 16, 2021 at 04:23:26PM +0100, Russell King wrote:
> > > On Wed, Jun 16, 2021 at 04:04:56PM +0100, Catalin Marinas wrote:
> > > > The simpler fix for flush_icache_range() is to disable preemption, read
> > > > a word in a cacheline to force any dirty lines on another CPU to be
> > > > evicted and then issue the D-cache maintenance (for those cache lines
> > > > which are still dirty on the current CPU).
> > > 
> > > Is just reading sufficient? If so, why do we do a read-then-write in
> > > the MPCore DMA cache ops? Don't we need the write to force exclusive
> > > ownership? If we don't have exclusive ownership of the dirty line,
> > > how can we be sure to write it out of the caches?
> > 
> > For cleaning (which is the case for I/D coherency), we only need reading
> > since we are fine with clean lines being left in the D-cache on other
> > CPUs. For invalidation, we indeed need to force the exclusive ownership,
> > hence the write.
> 
> Ah, I'm not sure the I-cache is broadcast in hardware on ARM11MPCore
> either. So fixing the D side won't be sufficient.

The other question is... do we bother to fix this.

Arnd tells me that the current remaining ARM11MPCore users are:
- CNS3xxx (where there is some martinal interest in the Gateworks
  Laguna platform)
- Similar for OXNAS
- There used to be the Realview MPCore tile - I haven't turned that on
  in ages, and it may be that the 3V cell that backs up the encryption
  keys is dead so it may not even boot.
- Not sure about the story with QEMU - Arnd doesn't think there would
  be a problem there as it may not model caches.

So it seems to come down to a question about CNS3xxx and OXNAS. If
these aren't being used, maybe we can drop ARM11MPCore support and
the associated platforms?

Linus, Krzysztof, Neil, any input?

Thanks.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!