netdev - Re: [Alacrityvm-devel] [PATCH v3 3/6] vbus: add a "vbus-proxy" bus model for vbus

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Date:	Wed, 19 Aug 2009 01:06:45 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Ira W. Snyder" <iws@...o.caltech.edu>
CC:	"Michael S. Tsirkin" <mst@...hat.com>,
	Gregory Haskins <gregory.haskins@...il.com>,
	kvm@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	alacrityvm-devel@...ts.sourceforge.net,
	Anthony Liguori <anthony@...emonkey.ws>,
	Ingo Molnar <mingo@...e.hu>,
	Gregory Haskins <ghaskins@...ell.com>
Subject: Re: [Alacrityvm-devel] [PATCH v3 3/6] vbus: add a "vbus-proxy" bus
 model for vbus_driver objects

On 08/19/2009 12:26 AM, Avi Kivity wrote:
>>
>> Off the top of my head, I would think that transporting userspace
>> addresses in the ring (for copy_(to|from)_user()) vs. physical addresses
>> (for DMAEngine) might be a problem. Pinning userspace pages into memory
>> for DMA is a bit of a pain, though it is possible.
>
>
> Oh, the ring doesn't transport userspace addresses.  It transports 
> guest addresses, and it's up to vhost to do something with them.
>
> Currently vhost supports two translation modes:
>
> 1. virtio address == host virtual address (using copy_to_user)
> 2. virtio address == offsetted host virtual address (using copy_to_user)
>
> The latter mode is used for kvm guests (with multiple offsets, 
> skipping some details).
>
> I think you need to add a third mode, virtio address == host physical 
> address (using dma engine).  Once you do that, and wire up the 
> signalling, things should work.


You don't need in fact a third mode.  You can mmap the x86 address space 
into your ppc userspace and use the second mode.  All you need then is 
the dma engine glue and byte swapping.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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