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Date:   Tue, 5 Feb 2019 08:24:07 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Thiago Jung Bauermann <bauerman@linux.ibm.com>,
        Christoph Hellwig <hch@lst.de>,
        Jason Wang <jasowang@redhat.com>,
        virtualization@lists.linux-foundation.org,
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        linux-kernel@vger.kernel.org,
        David Gibson <david@gibson.dropbear.id.au>,
        Alexey Kardashevskiy <aik@linux.ibm.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Ram Pai <linuxram@us.ibm.com>,
        Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Subject: Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is
 encrypted
Message-ID: <20190205072407.GA4311@lst.de>
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On Mon, Feb 04, 2019 at 04:38:21PM -0500, Michael S. Tsirkin wrote:
> It was designed to make, when set, as many guests as we can work
> correctly, and it seems to be successful in doing exactly that.
> 
> Unfortunately there could be legacy guests that do work correctly but
> become slow. Whether trying to somehow work around that
> can paint us into a corner where things again don't
> work for some people is a question worth discussing.

The other problem is that some qemu machines just throw passthrough
devices and virtio devices on the same virtual PCI(e) bus, and have a
common IOMMU setup for the whole bus / root port / domain.  I think
this is completely bogus, but unfortunately it is out in the field.

Given that power is one of these examples I suspect that is what
Thiago referes to.  But in this case the answer can't be that we
pile on hack ontop of another, but instead introduce a new qemu
machine that separates these clearly, and make that mandatory for
the secure guest support.