Re: [Qemu-devel] Anyone seeing huge slowdown launching qemu with Linux 2.6.35?

[Richard W.M. Jones]

On Tue, Aug 03, 2010 at 10:22:22PM +0300, Avi Kivity wrote:
>  On 08/03/2010 10:13 PM, Richard W.M. Jones wrote:
> >On Tue, Aug 03, 2010 at 09:43:39PM +0300, Avi Kivity wrote:
> >>libguestfs does not depend on an x86 architectural feature.
> >>qemu-system-x86_64 emulates a PC, and PCs don't have -kernel.  We
> >>should discourage people from depending on this interface for
> >>production use.
> >I really don't get this whole thing where we must slavishly
> >emulate an exact PC ...
> 
> This has two motivations:
> 
> - documented interfaces: we suck at documentation.  We seldom
> document.  Even when we do document something, the documentation is
> often inaccurate, misleading, and incomplete.  While an "exact PC"
> unfortunately doesn't exist, it's a lot closer to reality than, say,
> an "exact Linux syscall interface".  If we adopt an existing
> interface, we already have the documentation, and if there's a
> conflict between the documentation and our implementation, it's
> clear who wins (well, not always).
> 
> - preexisting guests: if we design a new interface, we get to update
> all guests; and there are many of them.  Whereas an "exact PC" will
> be seen by the guest vendors as well who will then add whatever
> support is necessary.

On the other hand we end up with stuff like only being able to add 29
virtio-blk devices to a single guest.  As best as I can tell, this
comes from PCI, and this limit required a bunch of hacks when
implementing virt-df.

These are reasonable motivations, but I think they are partially about
us:

We could document things better and make things future-proof.  I'm
surprised by how lacking the doc requirements are for qemu (compared
to, hmm, libguestfs for example).

We could demand that OSes write device drivers for more qemu devices
-- already OS vendors write thousands of device drivers for all sorts
of obscure devices, so this isn't really much of a demand for them.
In fact, they're already doing it.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine.  Supports Linux and Windows.
http://et.redhat.com/~rjones/virt-df/