[RFC] Using VMI with KVM

[Anthony Liguori <aliguori-NZpS4cJIG2HvQtjrzfazuQ@public.gmane.org>]

As I mentioned in a previous note, I've been playing around with using 
VMI to paravirtualize KVM.  There are a couple of interesting things 
about this that I thought I'd share what I currently have.

Unlike lhype or Xen, VMI simply redirects most sensitive operations into 
a block of code that's presented to the guest as a PCI option ROM.  The 
guest copies the ROM somewhere into it's address space and will call 
into the ROM whenever it needs to do something interesting.

A nice property of such a low-level interface is that it doesn't mandate 
very much.  With both Xen and lhype, you are forced to run guests in 
ring 1.  With VMI, you can happily run a guest in ring 0.  I think 
there's a lot of benefits to paravirtualizing a guest in ring 0 (no 
performance penalty for syscalls for instance).

It also means we can start out with a very simply "identity" ROM that 
implements all of the sensitive instructions as their native 
equivalents.  We have no real need to touch pushf's for instance so we 
don't need to paravirtualize these.  Instead, we can focus on things 
like PTE batching (which is something we can't do without 
paravirtualization).

At the moment, I have a working VMI ROM that uses native equivalents for 
all sensitive ops.  It's posted at the following location:

http://hg.codemonkey.ws/kvm-vmi

My plan was to implement IDE emulation entirely within the ROM using PIO 
VMI exits but I didn't discover until recently that PIO exits aren't in 
VMI yet :-)  My current plan is to try and implement PTE update batching 
but I need to spend some time figuring out how to setup the .ld file so 
that my data/bss segments aren't fubar (as they are at the moment).

You'll need my qemu-option-rom.diff and qemu-vmi.diff patches to be able 
to load the VMI ROM.  These are against CVS so they will fuzz a bit 
against KVM's QEMU.  You can get both of these patches from my qemu 
patch queue:

http://hg.codemonkey.ws/qemu-pq

You'll also need a VMI enabled kernel.  I'm currently using the VMI 
patches available in the paravirt_ops patch queue:

http://ozlabs.org/~rusty/paravirt/

I'm not hard set on using VMI but at the moment it seems like the 
easiest and most flexible approach to paravirtualizing KVM.  I'm 
certainly interested in what other people think though.

Regards,

Anthony Liguori

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