Re: [Qemu-devel] Looking for an easy way to exchange data bidirectional between host and guest (including some suggestion)

[Henrik Nordstrom]

On Mon, 6 Jun 2005, John R. Hogerhuis wrote:

> I can think of some reasons for a non-native file service that is
> instead built into QEMU:

vvfat?

> e) Scripts to be used across farms of QEMU virtual machines will have
> more commonalities even across different OSes. The event and content of
> a file popping up on a machine in a given directory is a very basic form
> of inter-process communication common in business operations.

Using something like vvfat for bi-directional IPC would be a very bad 
move. Most guests doesn't guarantee data written on a local drive gets 
commited to the drive in a timely fashion (can be anywhere from seconds to 
many minutes before the data is actually written).

If you need bi-directional communication with a running guest you should 
be using a networked filesystem. Most if not all other approaches will not 
play nicely with the guest.

Even if vvfat can provide what to the guest looks like FAT block 
device access to a directory of files on the host the directory must not 
be touched in any manner while qemu is accessing it. Providing this kind 
of emulation is complex enough as it is.

What vvfat does is that it emulates a FAT formatted block device from a 
directory of files on the host. Currently this emulation is read-only 
(mostly) but it is moving towards read-write access which will when 
completed allow the guest to not only read the files but also write new 
content. But it is important to understand that write access is far from 
trivial. The vvfat driver have to piece together the block level writes 
performed by the guest and reconstruct the filestructure to know what 
files to map this to on the host. It is not like the guest tells vvfat 
which file it is writing to.

Regards
Henrik