Re: [Qemu-devel] Re: [PATCH 15/15] Implement the bus structure for PAPR virtual IO

[David Gibson]

On Sun, Feb 13, 2011 at 10:08:23AM +0200, Blue Swirl wrote:
> On Sun, Feb 13, 2011 at 1:15 AM, Benjamin Herrenschmidt
> <address@hidden> wrote:
> > On Sun, 2011-02-13 at 00:52 +0200, Blue Swirl wrote:
> >> On Sat, Feb 12, 2011 at 11:00 PM, Benjamin Herrenschmidt
> >> <address@hidden> wrote:
> >> > On Sat, 2011-02-12 at 18:59 +0200, Blue Swirl wrote:
> >> >>
> >> >> Actually I don't quite understand the need for vty layer, why not use
> >> >> the chardev here directly?
> >> >
> >> > I'm not sure what you mean here...
> >>
> >> Maybe it would be reasonable to leave h_put_term_char to spapr_hcall.c
> >> instead of moving those to a separate file.
> >
> > Well, the VIO device instance gives the chardev instance which is all
> > nicely encapsulated inside spapr-vty... Also VIO devices tend to have
> > dedicated hcalls, not only VTY, so it makes a lot of sense to keep them
> > close to the rest of the VIO driver they belong to don't you think ?
> >
> > (Actually veth does, vscsi uses the more "generic" CRQ stuff which we've
> > added to the core VIO but you'll see that when we get those patches
> > ready, hopefully soon).
> 
> This is a bit of a special case, much like semihosting modes for m68k
> or ARM, or like MOL hacks which were removed recently. From QEMU point
> of view, the most natural way of handling this would be hypervisor
> implemented in the guest side (for example BIOS). Then the hypervisor
> would use normal IO (or virtio) to communicate with the host. If
> inside QEMU, the interface of the hypervisor to the devices needs some
> thought. We'd like to avoid ugly interfaces like vmmouse where a
> device probes CPU registers directly or spaghetti interfaces like
> APIC.

I really don't follow what you're saying here.  Running the hypervisor
in the guest, rather than emulating its effect in qemu seems like an
awful lot of complexity for no clear reason.

> > Actually, one thing I noticed is that the current patches David posted
> > still have a single function with a switch/case statement for hcalls.
> >
> > I'm not 100% certain what David long term plans are here, but in our
> > internal "WIP" tree, we've subsequently turned that into a mechanism
> > where any module can call powerpc_register_hypercall() to add hcalls.
> >
> > So if David intends to move the "upstream candidate" tree in that
> > direction, then naturally, the calls in spapr_hcall.c are going to
> > disappear in favor of a pair of powerpc_register_hypercall() locally in
> > the vty module.
> 
> Is the interface new design, or are you implementing what is used also
> on real HW?

The interface already exists on real HW.  It's described in the PAPR
document we keep mentioning.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson