linux-kernel - Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted

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Date:   Tue, 26 Mar 2019 12:53:59 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Jason Wang <jasowang@...hat.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        virtualization@...ts.linux-foundation.org,
        linuxppc-dev@...ts.ozlabs.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org,
        David Gibson <david@...son.dropbear.id.au>,
        Alexey Kardashevskiy <aik@...ux.ibm.com>,
        Paul Mackerras <paulus@...abs.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Ram Pai <linuxram@...ibm.com>,
        Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        eric.auger@...hat.com, joro@...tes.org,
        tnowicki@...iumnetworks.com, kevin.tian@...el.com,
        Lorenzo.Pieralisi@....com, bharat.bhushan@....com,
        Will.Deacon@....com, Robin.Murphy@....com, Marc.Zyngier@....com
Subject: Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted

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