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

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:

quoted

On Wed, Jun 16, 2021 at 04:23:26PM +0100, Russell King wrote:

quoted

On Wed, Jun 16, 2021 at 04:04:56PM +0100, Catalin Marinas wrote:

quoted

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.

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