Re: Configuration vs. compat hints [was Re: [Qemu-devel] [PATCHv3 03/13] qemu: add routines to manage PCI capabilities]

[Jamie Lokier <jamie@shareable.org>]

Avi Kivity wrote:
> >If management apps need to hard-code which slots are available on
> >different targets and different qemu versions, or restrictions on which
> >devices can use which slots, or knowledge that some devices can be
> >multi-function, or ... anything like that is just lame.
> >   
> 
> You can't abstract these things away.  If you can't put a NIC in slot 4, 
> and you have 7 slots, then you cannot have 7 NICs.  Having qemu allocate 
> the slot numbers does not absolve management from knowing this 
> limitation and preventing the user from creating a machine with 7 slots.
> 
> Likewise, management will have to know which devices are multi-function, 
> since that affects their hotpluggability.  Ditto if some slot if faster 
> than others, if you want to make use of this information you have to let 
> the upper layers know.
> 
> It could be done using an elaborate machine description that qemu 
> exposes to management coupled with a constraint solver that optimizes 
> the machine layout according to user specifications and hardware 
> limitations.  Or we could take the view that real life is not perfect 
> (especially where computers are involved), add some machine specific 
> knowledge, and spend the rest of the summer at the beach.

To be honest, an elaborate machine description is probably fine...

A fancy constraint solver is not required.  A simple one strikes me as
about as simple as what you'd hard-code anyway, but with fewer special
cases.

Note that the result can fail due to things like insufficient address
space for all the device BARs even when they _are_ in the right slots.
Especially if there are lots of slots, or bridges which can provide
unlimited slots.

That is arcane: device-dependent, CPU-dependent, machine-dependent,
RAM-size dependent (in a non-linear way), device-option-dependent and
probably QEMU-version-dependent too.

It would be nice if libvirt (et al) would prevent the user from
creating a VM with insufficient BAR space for that machine, but I'm
not sure how to do it sanely, without arcane knowledge getting about.

Maybe that idea of a .so shared by qemu and libvirt, to manipulate
device configurations, is a sane one after all.

-- Jamie