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Date:   Tue, 26 Mar 2019 12:53:59 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Jason Wang <jasowang@redhat.com>,
        Thiago Jung Bauermann <bauerman@linux.ibm.com>,
        virtualization@lists.linux-foundation.org,
        linuxppc-dev@lists.ozlabs.org, iommu@lists.linux-foundation.org,
        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>,
        eric.auger@redhat.com, joro@8bytes.org,
        tnowicki@caviumnetworks.com, kevin.tian@intel.com,
        Lorenzo.Pieralisi@arm.com, bharat.bhushan@nxp.com,
        Will.Deacon@arm.com, Robin.Murphy@arm.com, Marc.Zyngier@arm.com
Subject: Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted
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On Wed, Jan 30, 2019 at 08:44:27AM +0100, Christoph Hellwig wrote:
> On Tue, Jan 29, 2019 at 09:36:08PM -0500, Michael S. Tsirkin wrote:
> > This has been discussed ad nauseum. virtio is all about compatibility.
> > Losing a couple of lines of code isn't worth breaking working setups.
> > People that want "just use DMA API no tricks" now have the option.
> > Setting a flag in a feature bit map is literally a single line
> > of code in the hypervisor. So stop pushing for breaking working
> > legacy setups and just fix it in the right place.
> 
> I agree with the legacy aspect.  What I am missing is an extremely
> strong wording that says you SHOULD always set this flag for new
> hosts, including an explanation why.


So as far as power is concerned, IIUC the issue they are struggling with is
that some platforms do not support pass-through mode in the emulated IOMMU.
Disabling PLATFORM_ACCESS is so far a way around that, unfortunately just for
virtio devices.  I would like virtio-iommu to be able to address that need as
well.

-- 
MST