From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 1/2] mm: move force_dma_unencrypted() to mem_encrypt.h
Date: Thu, 20 Feb 2020 17:31:35 +0100
Message-ID: <20200220163135.GA13192@lst.de>
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To: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: linux-s390@vger.kernel.org, Janosch Frank <frankja@linux.ibm.com>, "Michael S. Tsirkin" <mst@redhat.com>, Cornelia Huck <cohuck@redhat.com>, Ram Pai <linuxram@us.ibm.com>, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Halil Pasic <pasic@linux.ibm.com>, iommu@lists.linux-foundation.org, David Gibson <david@gibson.dropbear.id.au>, Michael Mueller <mimu@linux.ibm.com>, "Lendacky,
 Thomas" <Thomas.Lendacky@amd.com>, Viktor Mihajlovski <mihajlov@linux.ibm.com>, Robin Murphy <robin.murphy@arm.com>, Christoph Hellwig <hch@lst.de>, Marek Szyprowski <m.szyprowski@samsung.com>
List-Id: virtualization@lists.linuxfoundation.org

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.