I'm running QEMU with kqemu on my old 700MHz laptop.
User-mode stuff is slowed down only slightly. This command line:
time for x in $(seq 10000); do :; :; :; :; done
takes 1.17 1.19 1.20 1.22 user seconds in emulation and 1.13 1.13 1.14
1.14 user seconds outside QEMU.
However, it takes about 100ms of system time in place of about 10ms.
(The `-kernel-kqemu` flag may solve this; haven't measured.)
I had some kind of keyboard problem when I ran QEMU 0.8.2-4etch1 with
`-snapshot`. Like, the keyboard just didn't work. That problem went
away when I built QEMU 0.9.1 from source and started using that, but I
still can't use `-snapshot` and `-loadvm` together.
Networking: `tap`
-----------------
This was a bad idea (for me).
By default, QEMU uses `user` networking, which proxies network
connections through normal sockets, like `slipknot` or `slirp` or
`term`. (In fact, it uses `slirp`.) I thought this didn't give me a
way to talk to it over the network (for example, if I'm running a web
server on it).
So I thought `-net tap` could help with this, but it has some
drawbacks. It requires running QEMU as root, and then the network
interface on the emulated machine needs to be configured statically,
e.g. in `/etc/network/interfaces`, since `-net tap` doesn't provide
DHCP by default. And then you have to set up IP masquerading, more or
less as follows:
qemu -net nic -net tap,script=ifup "$image"
In file `ifup`:
set -e
/sbin/ifconfig "$1" 172.20.0.1
echo 1 > /proc/sys/net/ipv4/ip_forward
/sbin/iptables -t nat -A POSTROUTING --source 172.20.0.0/24 -j MASQUERADE
This does actually work, but you have to configure the network stuff
inside of QEMU: IP address, netmask, default gateway, and worst of
all, DNS server. And I think it might allow other people on your LAN
to masquerade through you.
What would be ideal would be bridging the virtual interface to my real
Ethernet interface, but I never got around to doing this.
Networking: `-redir`
--------------------
It turns out there's an easier way. I can use the default `user`
networking, and if I have a web server on the emulated host on port
8080, I can say
qemu -redir tcp:8000::8080 "$image"
and connect my web browser to <http://localhost:8000/>.
This works beautifully. The one downside I've found is that if you're
using `qemu -loadvm`, the inner virtual machine has to re-request DHCP
before the redirection works.
Startup: `-loadvm`
------------------
Bootup takes an annoyingly long time. But, if you don't regularly
have any permanent changes you want to save, you can use the `savevm`
command to save an image of the virtual machine state after a boot,
and then use `qemu -loadvm` to start QEMU in the already-booted state.
