linux-kernel - Re: [PATCH 1/2] mm: move force_dma_unencrypted() to mem

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Date:   Thu, 20 Feb 2020 17:23:20 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Christoph Hellwig <hch@....de>, Halil Pasic <pasic@...ux.ibm.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        linux-s390@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Janosch Frank <frankja@...ux.ibm.com>,
        Viktor Mihajlovski <mihajlov@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Ram Pai <linuxram@...ibm.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        David Gibson <david@...son.dropbear.id.au>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        Michael Mueller <mimu@...ux.ibm.com>
Subject: Re: [PATCH 1/2] mm: move force_dma_unencrypted() to mem_encrypt.h



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