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

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Message-ID: <20190812121532.GB9405@lst.de>
Date:   Mon, 12 Aug 2019 14:15:32 +0200
From:   Christoph Hellwig <hch@....de>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Christoph Hellwig <hch@....de>, Ram Pai <linuxram@...ibm.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        virtualization@...ts.linux-foundation.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Jason Wang <jasowang@...hat.com>,
        David Gibson <david@...son.dropbear.id.au>,
        Alexey Kardashevskiy <aik@...ux.ibm.com>,
        Paul Mackerras <paulus@...abs.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is
 encrypted

On Sun, Aug 11, 2019 at 04:55:27AM -0400, Michael S. Tsirkin wrote:
> On Sun, Aug 11, 2019 at 07:56:07AM +0200, Christoph Hellwig wrote:
> > So we need a flag on the virtio device, exposed by the
> > hypervisor (or hardware for hw virtio devices) that says:  hey, I'm real,
> > don't take a shortcut.
> 
> The point here is that it's actually still not real. So we would still
> use a physical address. However Linux decides that it wants extra
> security by moving all data through the bounce buffer.  The distinction
> made is that one can actually give device a physical address of the
> bounce buffer.

Sure.  The problem is just that you keep piling hacks on top of hacks.
We need the per-device flag anyway to properly support hardware virtio
device in all circumstances.  Instead of coming up with another ad-hoc
hack to force DMA uses implement that one proper bit and reuse it here.