Date: Fri, 20 Nov 2015 08:56:46 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Sebastian Ott <sebott@linux.vnet.ibm.com>,
        linux-s390 <linux-s390@vger.kernel.org>,
        Cornelia Huck <cornelia.huck@de.ibm.com>,
        Joerg Roedel <jroedel@suse.de>,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        Christoph Hellwig <hch@lst.de>, David Woodhouse <dwmw2@infradead.org>,
        KVM <kvm@vger.kernel.org>
Subject: Re: [PATCH v3 0/3] virtio DMA API core stuff
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On Thu, Nov 19, 2015 at 01:59:05PM -0800, Andy Lutomirski wrote:
> On Nov 19, 2015 5:45 AM, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> >
> > On Tue, Oct 27, 2015 at 11:38:57PM -0700, Andy Lutomirski wrote:
> > > This switches virtio to use the DMA API unconditionally.  I'm sure
> > > it breaks things, but it seems to work on x86 using virtio-pci, with
> > > and without Xen, and using both the modern 1.0 variant and the
> > > legacy variant.
> >
> > So thinking hard about it, I don't see any real drawbacks to making this
> > conditional on a new feature bit, that Xen can then set..
> 
> Can you elaborate?  If I run QEMU, hosting Xen, hosting Linux, and the
> virtio device is provided by QEMU, then how does Xen set the bit?

You would run QEMU with the appropriate flag. E.g.
-global virtio-pci,use_platform_dma=on

> Similarly, how would Xen set the bit for a real physical device?
> 
> 
> --Andy

There's no need to set bits for physical devices I think: from security
point of view, using them from a VM isn't very different from using them
from host.


-- 
MST
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