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

[Corey Minyard <tcminyard@gmail.com>]

I think we are getting a little out of hand here, and we are mixing up 
concepts :).

There are lots of things IPMI *can* do (including serial access, VGA 
snooping, LAN access, etc.) but I don't see any value it that.  The main 
thing here is to emulate the interface to the guest.  OOB management is 
really more appropriately handled with libvirt.  How the BMC integrates 
into the hardware varies a *lot* between systems, but it's really kind 
of irrelevant.  (Well, almost irrelevant, BMCs can provide a direct I2C 
messaging capability, and that may matter.)

A guest can have one (or more) of a number of interfaces (that are all 
fairly bad, unfortunately).  The standard ones are called "KCS", "BT" 
and "SMIC" and they generally are directly on the ISA bus, but are in 
memory on non-x86 boxes (and on some x86 boxes) and sometimes on the PCI 
bus.  Some systems also have interfaces over I2C, but that hasn't really 
caught on.  Others have interfaces over serial ports, and that 
unfortunately has caught on in the ATCA world.  And there are at least 3 
different basic types of serial port interfaces with sub-variants :(.  
I'm not sure what the USB rndis device is, but I'll look at it.  But 
there is no IPMI over USB.

The big things a guest can do are sensor management, watchdog timer, 
reset, and power control.  In complicated IPMI-based systems like ATCA, 
a guest may want to send messages through its local IPMI controller to 
other guest's IPMI controllers or to a central BMC that runs an entire 
chassis of systems.  So that may need to be supported, depending on what 
people want to do and how hard they want to work on it.

My proposal is to start small, with just a local interface, watchdog 
timer, sensors and power control.  But have an architecture that would 
allow external LAN access, tying BMCs in different qemu instances 
together, perhaps serial over IPMI, and other things of that nature.

-corey


On 05/07/2012 10:21 AM, Anthony Liguori wrote:
> 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