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Subject: Re: [PATCH 1/2] mm: move force_dma_unencrypted() to mem_encrypt.h
To:     Christoph Hellwig <hch@lst.de>, Halil Pasic <pasic@linux.ibm.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        linux-s390@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
        Janosch Frank <frankja@linux.ibm.com>,
        Viktor Mihajlovski <mihajlov@linux.ibm.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Ram Pai <linuxram@us.ibm.com>,
        Thiago Jung Bauermann <bauerman@linux.ibm.com>,
        David Gibson <david@gibson.dropbear.id.au>,
        "Lendacky, Thomas" <Thomas.Lendacky@amd.com>,
        Michael Mueller <mimu@linux.ibm.com>
References: <20200220160606.53156-1-pasic@linux.ibm.com>
 <20200220160606.53156-2-pasic@linux.ibm.com> <20200220161146.GA12709@lst.de>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
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Date:   Thu, 20 Feb 2020 17:23:20 +0100
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On 20.02.20 17:11, Christoph Hellwig wrote:
> On Thu, Feb 20, 2020 at 05:06:05PM +0100, Halil Pasic wrote:
>> Currently force_dma_unencrypted() is only used by the direct
>> implementation of the DMA API, and thus resides in dma-direct.h. But
>> there is nothing dma-direct specific about it: if one was -- for
>> whatever reason -- to implement custom DMA ops that have to in the
>> encrypted/protected scenarios dma-direct currently deals with, one would
>> need exactly this kind of information.
> 
> I really don't think it has business being anywhre else, and your completely
> bogus second patch just proves the point.

From a users perspective it makes absolutely perfect sense to use the
bounce buffers when they are NEEDED. 
Forcing the user to specify iommu_platform just because you need bounce buffers
really feels wrong. And obviously we have a severe performance issue
because of the indirections.

Now: I understand that you want to get this fixes differently, but maybe you 
could help to outline how this could be fixed proper. 

Christian