On 03/25/2012 04:36 PM, Anthony Liguori wrote:
Apart from the command line length, it confuses configuration with definition.
There is no distinction with what we have today. Our configuration file basically corresponds to command line options and as there is no distinction in command line options, there's no distinction in the configuration format.
We don't have command line options for defining, only configuring. Again, defining = #define Configuring = modifying current instance
target-x86_64-cpus.cfg does not configure qemu for anything, it's merely the equivalent of
#define westmere (x86_def_t) { ... } #define nehalem (x86_def_t) { ... } #define bulldozer (x86_def_t) { ... } // for PC
so it should be read at each invocation. On the other hand, pc.cfg and westmere.cfg (as used previously) are shorthand for
machine = (QEMUMachine) { ... }; cpu = (x86_def_t) { ... };
so they should only be read if requested explicitly (or indirectly).
This doesn't make a lot of sense to me. Here's what I'm proposing:
1) QEMU would have a target-x86_64-cpu.cfg.in that is installed by default in /etc/qemu. It would contain:
[system] # Load default CPU definitions readconfig = @DATADIR@/target-x86_64-cpus.cfg
2) target-x86_64-cpus.cfg would be installed to @DATADIR@ and would contain:
[cpudef] name = "Westmere" ...
This has the following properties:
A) QEMU has no builtin notion of CPU definitions. It just has a "cpu factory". -cpudef will create a new class called Westmere that can then be enumerated through qom-type-list and created via qom-create.
B) A management tool has complete control over cpu definitions without modifying the underlying filesystem. -nodefconfig will prevent it from loading and the management tool can explicitly load the QEMU definition (via -readconfig, potentially using a /dev/fd/N path) or it can define it's own cpu definitions.
Why does -nodefconfig affect anything? The file defines westmere as an alias for a grab bag of options. Whether it's loaded or not is immaterial, unless someone uses one of the names within.
C) This model maps to any other type of class factory. Machines will eventually be expressed as a class factory. When we implement this, we would change the default target-x86_64-cpu.cfg to:
[system] # Load default CPU definitions readconfig = @DATADIR@/target-x86_64-cpus.cfg # Load default machines readconfig = @DATADIR@/target-x86_64-machines.cfg
A machine definition would look like:
[machinedef] name = pc-0.15 virtio-blk.class_code = 32 ...
Loading a file based on -cpu doesn't generalize well unless we try to load a definition for any possible QOM type to find the class factory for it. I don't think this is a good idea.
Why not load all class factories? Just don't instantiate any objects. Otherwise, the meaning of -nodefconfig changes as more stuff is moved out of .c and into .cfg.
The reasoning is, loading target-x86_64-cpus.cfg does not alter the current instance's configuration, so reading it doesn't violate -nodefconfig.
I think we have a different view of what -nodefconfig does.
We have a couple options today:
-nodefconfig
Don't read the default configuration files. By default, we read /etc/qemu/qemu.cfg and /etc/qemu/target-$(ARCH).cfg
The latter seems meaningless to avoid reading. It's just a set of #defines, what do you get by not reading it?
In my target-$(ARCH).cfg, I have:
[machine] enable-kvm = "on"
Which means I don't have to use -enable-kvm anymore. But if you look at a tool like libguestfs, start up time is the most important thing so avoiding unnecessary I/O and processing is critical.
So this is definitely configuration (applies to the current instance) as opposed to target-x86_64.cfg, which doesn't.
-nodefaults
Don't create default devices.
-vga none
Don't create the default VGA device (not covered by -nodefaults).
With these two options, the semantics you get an absolutely minimalistic instance of QEMU. Tools like libguestfs really want to create the simplest guest and do the least amount of processing so the guest runs as fast as possible.
It does suck a lot that this isn't a single option. I would much prefer -nodefaults to be implied by -nodefconfig. Likewise, I would prefer that -nodefaults implied -vga none.
I don't have a qemu.cfg so can't comment on it, but in what way does reading target-x86_64.cfg affect the current instance (that is, why is -nodefconfig needed over -nodefaults -vga look-at-the-previous-option?)
It depends on what the user configures it to do.
How? As far as I can tell, the only difference is that -nodefconfig -cpu westmere will error out instead of working. But if you don't supply -cpu westmere, the configuration is identical. -- error compiling committee.c: too many arguments to function