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Date: Sat, 11 Jul 2009 01:01:16 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] bidirectional data exchange between guest and host
	without network
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To: Lennart Sorensen <lsorense@csclub.uwaterloo.ca>
Cc: Anthony Lannuzel <anthony.lannuzel@hynesim.org>, qemu-devel@nongnu.org

Lennart Sorensen wrote:
> On Wed, Jul 08, 2009 at 12:03:20PM +0200, Anthony Lannuzel wrote:
> > I tried using a fat partition with the "sync" mount option, to avoid
> > caching on both sides (still using virtio on /dev/sda8 on the host and
> > /dev/vda on the guest).
> 
> sync only affects write caching, it does nothing for read caching,
> which is where your real problem is.
> 
> > The host now reads data written by the guest but the guest does not see
> > the host data.
> > 
> > Is there anything related to virtio that prevents this from working, as I
> > think the mount option provides me with a filesystem that fulfils the
> > conditions you just told me ?
> 
> Just the fact any sane OS does read caching is what makes it not work.
> Filesystems are NOT going to do this for you.  You can not share
> filesystems like that unless explicitly designed for it.

Many guest OSes have O_DIRECT or equivalent.  That bypasses read and
write caching, and can be used for direct access to the underlying
block device.

You can share data over a block device that way, by _not_ using a
filesystem, only using your own protocol on the block filesystem.

-- Jamie