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

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Message-Id: <875ztzxvw2.fsf@morokweng.localdomain>
Date:   Mon, 04 Feb 2019 16:15:41 -0200
From:   Thiago Jung Bauermann <bauerman@...ux.ibm.com>
To:     Christoph Hellwig <hch@....de>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.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>
Subject: Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted


Christoph Hellwig <hch@....de> writes:

> 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.

My understanding of ACCESS_PLATFORM is that it means "this device will
behave in all aspects like a regular device attached to this bus". Is
that it? Therefore it should be set because it's the sane thing to do?

--
Thiago Jung Bauermann
IBM Linux Technology Center