From mboxrd@z Thu Jan  1 00:00:00 1970
From: Gregory Haskins <gregory.haskins@gmail.com>
Subject: Re: [PATCHv5 3/3] vhost_net: a kernel-level virtio server
Date: Thu, 17 Sep 2009 00:13:06 -0400
Message-ID: <4AB1B752.4010505@gmail.com>
References: <4AAF8A03.5020806@redhat.com> <4AAF909F.9080306@gmail.com> <4AAF95D1.1080600@redhat.com> <4AAF9BAF.3030109@gmail.com> <4AAFACB5.9050808@redhat.com> <4AAFF437.7060100@gmail.com> <4AB0A070.1050400@redhat.com> <4AB0CFA5.6040104@gmail.com> <4AB0E2A2.3080409@redhat.com> <4AB0F1EF.5050102@gmail.com> <20090917035717.GB3088@redhat.com>
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Cc: Avi Kivity <avi@redhat.com>, "Ira W. Snyder" <iws@ovro.caltech.edu>,
 netdev@vger.kernel.org, virtualization@lists.linux-foundation.org,
 kvm@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@elte.hu,
 linux-mm@kvack.org, akpm@linux-foundation.org, hpa@zytor.com,
 Rusty Russell <rusty@rustcorp.com.au>,
 s.hetze@linux-ag.com, alacrityvm-devel@lists.sourceforge.net
To: "Michael S. Tsirkin" <mst@redhat.com>
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Michael S. Tsirkin wrote:
> On Wed, Sep 16, 2009 at 10:10:55AM -0400, Gregory Haskins wrote:
>>> There is no role reversal.
>> So if I have virtio-blk driver running on the x86 and vhost-blk device=

>> running on the ppc board, I can use the ppc board as a block-device.
>> What if I really wanted to go the other way?
>=20
> It seems ppc is the only one that can initiate DMA to an arbitrary
> address, so you can't do this really, or you can by tunneling each
> request back to ppc, or doing an extra data copy, but it's unlikely to
> work well.
>=20
> The limitation comes from hardware, not from the API we use.

Understood, but presumably it can be exposed as a sub-function of the
ppc's board's register file as a DMA-controller service to the x86.
This would fall into the "tunnel requests back" category you mention
above, though I think "tunnel" implies a heavier protocol than it would
actually require.  This would look more like a PIO cycle to a DMA
controller than some higher layer protocol.

You would then utilize that DMA service inside the memctx, and it the
rest of vbus would work transparently with the existing devices/drivers.

I do agree it would require some benchmarking to determine its
feasibility, which is why I was careful to say things like "may work"
;).  I also do not even know if its possible to expose the service this
way on his system.  If this design is not possible or performs poorly, I
admit vbus is just as hosed as vhost in regard to the "role correction"
benefit.

Kind Regards,
-Greg


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