Re: [Qemu-devel] Adding an IPMI BMC device to KVM

[Anthony Liguori]

On 05/07/2012 10:11 AM, Avi Kivity wrote:
> On 05/07/2012 05:55 PM, Anthony Liguori wrote:
> > > > For all intents and purposes, the BMC/RSA is a separate physical
> > > > machine.
> > >
> > > That's true for any other card on a machine.
> >
> >
> > It has a separate power source for all intents and purposes.  If you
> > think of it in QOM terms, what connects the nodes together ultimately
> > is the "Vcc" pin that travels across all devices.  The RTC is a little
> > special because it has a battery backed CMOS/clock but it's also
> > handled specially.
>
> And we fail to emulate it correctly as well, wrt. alarms.
>
> > The BMC does not share Vcc.  It's no different than a separate
> > physical box.  It just shares a couple buses.
>
> It controls the main power place, reset line, can read VGA and emulate
> keyboard, seems pretty well integrated.

Emulating the keyboard is done through USB.  How the VGA thing works is very 
vendor dependent.  The VGA snooping can happen as part of the display path 
(essentially connected via a VGA cable) or it can be a side-band using a special 
graphics adapter.  I think QEMU VNC emulation is a pretty good analogy actually.

> > > That is one way to do it.  Figure out the interactions between two
> > > different parts in a machine, define an interface for them to
> > > communicate, and split them into two processes.  We don't usually do
> > > that; I believe your motivation is that the two have different power
> > > domains (but then so do NICs with wake-on-LAN support).
> >
> > The power still comes from the PCI bus.
>
> Maybe.  But it's on when the rest of the machine is off.  So Vcc is not
> shared.

That's all plumbed through the PCI bus FWIW.

> > Think of something like a blade center.  Each individual blade does
> > not have it's own BMC.  There's a single common BMC that provides an
> > IPMI interface for all blades.  Yet each blade still sees an IPMI
> > interface via a USB rndis device.
> >
> > You can rip out the memory, PCI devices, etc. from a box while the
> > Power is in and the BMC will be unaffected.
> >
> > > > At any rate, you would have some sort of virtual hardware device that
> > > > essentially spoke QMP to the slave instance.  You could just do
> > > > virtio-serial and call it a day actually.
> > >
> > > Sorry I lost you.  Which is the master and which is the slave?
> >
> > The BMC is the master, system being controlled is the slave.
>
> Ah okay.  It also has to read the VGA output (say via vnc) and supply
> keyboard input (via sendkey).

Right, QMP + VNC is a pretty accurate analogy.

> > > > It really boils down to what you are trying to do.  If you want to
> > > > just get some piece of software working that expects to do IPMI, the
> > > > easiest thing to do is run IPMI in the host and use a USB rndis
> > > > interface to interact with it.
> > >
> > > That would be most strange.  A remote client connecting to the IPMI
> > > interface would control the power level of the host, not the guest.
> >
> > IPMI with a custom backend is what I mean.  That's what I mean by an
> > IPMI ->  libvirt bridge.  You could build a libvirt client that exposes
> > an IPMI interface and when you issue IPMI commands, it translate it to
> > libvirt operations.
> >
> > This can run as a normal process on the host and then network it to
> > the guest via an emulated USB rndis device.  Existing software on the
> > guest shouldn't be able to tell the difference as long as it doesn't
> > try to use I2C to talk to the BMC.
>
> I still like the single process solution, it is more in line with the
> rest of qemu and handles live migration better.

Two QEMU processes could be migrated in unison if you really wanted to support 
that...

With qemu-system-mips/sh4 you could probably even run the real BMC software 
stack if you were so inclined :-)

> But even better would
> be not to do this at all, and satisfy the remote management requirements
> using the existing tools.

Right.

Regards,

Anthony Liguori