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

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Message-ID: <20190130074427.GA29516@lst.de>
Date:   Wed, 30 Jan 2019 08:44:27 +0100
From:   Christoph Hellwig <hch@....de>
To:     "Michael S. Tsirkin" <mst@...hat.com>
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, Christoph Hellwig <hch@....de>,
        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>
Subject: Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is
 encrypted

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.