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Subject: Re: [PATCH 2/8] powerpc/dma: properly wire up the unmap_page and
 unmap_sg methods
To: Christoph Hellwig <hch@lst.de>
References: <20181216171951.31306-1-hch@lst.de>
 <20181216171951.31306-3-hch@lst.de>
 <ea83ba21-feee-9776-ab0a-5bc78b237e4c@c-s.fr> <20181217073434.GC1933@lst.de>
From: Christophe Leroy <christophe.leroy@c-s.fr>
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Date: Mon, 17 Dec 2018 08:39:17 +0100
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Le 17/12/2018 à 08:34, Christoph Hellwig a écrit :
> On Mon, Dec 17, 2018 at 07:41:54AM +0100, Christophe Leroy wrote:
>>
>>
>> Le 16/12/2018 à 18:19, Christoph Hellwig a écrit :
>>> The unmap methods need to transfer memory ownership back from the device
>>> to the cpu by identical means as dma_sync_*_to_cpu.  I'm not sure powerpc
>>> needs to do any work in this transfer direction, but given that it does
>>> invalidate the caches in dma_sync_*_to_cpu already we should make sure
>>> we also do so on unmapping.
>>
>> Why do we have to do that on unmapping ? If we are unmapping it means we
>> are retiring the area, so why would we need to use CPU for syncing an area
>> than won't be used anymore ?
> 
> So in general we need the cache maintaince only at map time if the CPUs
> gurantee to never ѕpeculate into memory that might be under DMA, which
> for modern CPUs is increasingly rate.  If the CPUs could speculate into
> the area that was DMA mapped we need to do another round of cache
> maintainance on unmap to make sure we really do not have any data
> in the cache.  powerpc currently does that for dma_sync_*_to_cpu, but
> not for the unmap case, which not only looks odd but also seems to be
> worked around in drivers (see the ppc44x crypto patch).
> 
> Note that there are some way to optimize the amount of cache flushing
> done even when the cpu speculates, see:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arc/mm/dma.c#n93
> 
> But for that I need help with people that actually understand the
> non-coherent powerpc SOCs and who can test it.  For now this patch
> is conservative.
> 

I can help you with powerpc 8xx actually.

Christophe