netdev - Re: [PATCHv5 3/3] vhost_net: a kernel-level virtio server

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Message-Id: <200909161722.37606.arnd@arndb.de>
Date:	Wed, 16 Sep 2009 17:22:37 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Gregory Haskins <gregory.haskins@...il.com>,
	Avi Kivity <avi@...hat.com>,
	"Ira W. Snyder" <iws@...o.caltech.edu>, netdev@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, mingo@...e.hu, linux-mm@...ck.org,
	akpm@...ux-foundation.org, hpa@...or.com,
	Rusty Russell <rusty@...tcorp.com.au>, s.hetze@...ux-ag.com,
	alacrityvm-devel@...ts.sourceforge.net
Subject: Re: [PATCHv5 3/3] vhost_net: a kernel-level virtio server

On Wednesday 16 September 2009, Michael S. Tsirkin wrote:
> On Wed, Sep 16, 2009 at 04:57:42PM +0200, Arnd Bergmann wrote:
> > On Tuesday 15 September 2009, Michael S. Tsirkin wrote:
> > > Userspace in x86 maps a PCI region, uses it for communication with ppc?
> > 
> > This might have portability issues. On x86 it should work, but if the
> > host is powerpc or similar, you cannot reliably access PCI I/O memory
> > through copy_tofrom_user but have to use memcpy_toio/fromio or readl/writel
> > calls, which don't work on user pointers.
> > 
> > Specifically on powerpc, copy_from_user cannot access unaligned buffers
> > if they are on an I/O mapping.
> > 
> We are talking about doing this in userspace, not in kernel.

Ok, that's fine then. I thought the idea was to use the vhost_net driver
to access the user memory, which would be a really cute hack otherwise,
as you'd only need to provide the eventfds from a hardware specific
driver and could use the regular virtio_net on the other side.

	Arnd <><
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