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Date:   Thu, 20 Feb 2020 17:31:35 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Christian Borntraeger <borntraeger@de.ibm.com>
Cc:     Christoph Hellwig <hch@lst.de>, Halil Pasic <pasic@linux.ibm.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
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        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>
Subject: Re: [PATCH 1/2] mm: move force_dma_unencrypted() to mem_encrypt.h
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On Thu, Feb 20, 2020 at 05:23:20PM +0100, Christian Borntraeger wrote:
> >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.

The point is that the user should not have to specify iommu_platform.
We need to make sure any new hypervisor (especially one that might require
bounce buffering) always sets it, as was a rather bogus legacy hack
that isn't extensibe for cases that for example require bounce buffering.