[Lguest] [patch 37/43] lguest: Virtio helper routines for a descriptor ringbuffer implementation

[Rusty Russell]

On Sun, 2007-09-30 at 19:03 +0200, Avi Kivity wrote:
> rusty at rustcorp.com.au wrote:
> 
> > These helper routines supply most of the virtqueue_ops for hypervisors
> > which want to use a ring for virtio.  Unlike the previous lguest
> > implementation:
> >
> > 1) The rings are variable sized (2^n-1 elements).
> > 2) They have an unfortunate limit of 65535 bytes per sg element.
> > 3) The page numbers are always 64 bit (PAE anyone?)
> > 4) They no longer place used[] on a separate page, just a separate
> >    cacheline.
> > 5) We do a modulo on a variable.  We could be tricky if we cared.
> > 6) Interrupts and notifies are suppressed using flags within the rings.
> >
> > Users need only get the ring pages and provide a notify hook (KVM
> > wants the guest to allocate the rings, lguest does it sanely).
> 
> This fixes an ABI.  I don't think we have enough experience with this to 
> set an ABI for 2.6.24.  I also owe you a patch for scatter/gather 
> indirection instead of chaining.
> 
> It is okay to put this in as long as 2.6.24 drivers won't try to talk to 
> 2.6.25+ hosts (which presumably will have a stable ABI).  We could do 
> this by having "stable API" feature bit which the new drivers will 
> require, and an "2.6.24 ABI" feature bit which the current drivers will 
> require, but the whole config interface itself is too new for this in my 
> opinion.

Yes, but this is what lguest is for, to play without fixing an ABI.  I
fully expect incompatible tweaks before KVM causes the ABI to nail down.

And it's going to be far easier to manage lguest-kvm cooperation with
this in tree than outside.

Cheers,
Rusty.