Re: [Qemu-devel] Managing architectural restrictions with -device and libvirt

[Mark Cave-Ayland]

On 05/07/17 16:46, Markus Armbruster wrote:

>>>> I've been working on a patchset that brings the sun4u machine on
>>>> qemu-system-sparc64 much closer to a real Ultra 5, however due to
>>>> various design restrictions I need to be able to restrict how devices
>>>> are added to the machine with -device.
>>>>
>>>> On a real Ultra 5, the root PCI bus (sabre) has 2 PCI bridges (simba A
>>>> and simba B) with the onboard devices attached to simba A with 2 free
>>>> slots, and an initially empty simba B.
>>>>
>>>> Firstly, is it possible to restrict the machine so that devices cannot
>>>> be directly plugged into the root PCI bus, but only behind one of the
>>>> PCI bridges? There is also an additional restriction in that slot 0
>>>> behind simba A must be left empty to ensure that the ebus (containing
>>>> the onboard devices) is the first device allocated.
>>>
>>> I figure sabre, simba A, simba B and the onboard devices attached to
>>> simba A are all created by MachineClass init().
>>
>> Yes that is effectively correct, although the Simba devices are created
>> as part of the PCI host bridge (apb) creation in pci_apb_init().
> 
> Anything that runs within init() counts as "created by init()".

Okay, in that case we should be fine here.

>>> What device provides "the ebus", and how is it created?
>>
>> It's actually just an ISA bus, so the ebus device is effectively a
>> PCI-ISA bridge for legacy devices.
> 
> Is this bridge created by init()?

Yes, it too is called via the machine init function.

>>> Can you provide a list of all onboard PCI devices and how they are
>>> connected?  Diagram would be best.
>>
>> I can try and come up with something more concise later, however I can
>> quickly give you the OpenBIOS DT from my WIP patchset if that helps:
>>
>> 0 > show-devs
>> ffe1bf38 /
>> ffe1c110 /aliases
>> ffe1c238 /openprom (BootROM)
>> ffe26b50 /openprom/client-services
>> ffe1c4f0 /options
>> ffe1c5d0 /chosen
>> ffe1c710 /builtin
>> ffe1c838 /builtin/console
>> ffe26618 /packages
>> ffe28640 /packages/cmdline
>> ffe28890 /packages/disk-label
>> ffe2c8d8 /packages/deblocker
>> ffe2cef0 /packages/grubfs-files
>> ffe2d300 /packages/sun-parts
>> ffe2d718 /packages/elf-loader
>> ffe2b210 /address@hidden,0 (memory)
>> ffe2b370 /virtual-memory
>> ffe2d878 /address@hidden,0 (pci)
>> ffe2e1a8 /address@hidden,0/address@hidden,1 (pci)
>> ffe2e960 /address@hidden,0/address@hidden,1/address@hidden
>> ffe2f1b0 /address@hidden,0/address@hidden,1/address@hidden/address@hidden
>> ffe2f328 /address@hidden,0/address@hidden,1/address@hidden/address@hidden 
>> (block)
>> ffe2f878 /address@hidden,0/address@hidden,1/address@hidden/address@hidden 
>> (serial)
>> ffe2fc08 /address@hidden,0/address@hidden,1/address@hidden/address@hidden 
>> (8042)
>> ffe2fe00 
>> /address@hidden,0/address@hidden,1/address@hidden/address@hidden/address@hidden
>>  (serial)
>> ffe301b0 /address@hidden,0/address@hidden,1/address@hidden,1 (network)
>> ffe307c8 /address@hidden,0/address@hidden,1/QEMU,address@hidden (display)
>> ffe31e40 /address@hidden,0/address@hidden,1/address@hidden (ide)
>> ffe32398 /address@hidden,0/address@hidden,1/address@hidden/address@hidden 
>> (ide)
>> ffe32678 /address@hidden,0/address@hidden,1/address@hidden/address@hidden 
>> (ide)
>> ffe32910 
>> /address@hidden,0/address@hidden,1/address@hidden/address@hidden/address@hidden
>>  (block)
>> ffe32f98 /address@hidden,0/address@hidden (pci)
>> ffe336e8 /SUNW,UltraSPARC-IIi (cpu)
>>  ok
>>
>> For comparison you can see the DT from a real Ultra 5 here:
>> http://www.pearsonitcertification.com/articles/article.aspx?p=440286&seqNum=7
>>
>>> The real sabre has two slots, and doesn't support hot (un)plug.  Can we
>>> simply model that?  If yes, the root PCI bus is full after init(), and
>>> remains full.  Takes care of "cannot directly plugged into the root PCI
>>> bus".
>>
>> Right. So what you're saying is that if we add the 2 simba devices to
>> the sabre PCI host bridge during machine init and then mark the sabre
>> PCI root bus as not hotplug-able then that will prevent people adding
>> extra devices from the command line via -device? I will see if I can
>> find time to try this later this evening.
> 
> No.  Marking the bus "not hotpluggable" only prevents *hotplug*,
> i.e. plug/unplug after machine initialization completed, commonly with
> device_add.  -device is *cold* plug; it happens during machine
> initialization.
> 
> However, if you limit sabre's bus to two slots (modelling real hardware
> faithfully), then you can't cold plug anything (there's no free slot).
> If you additionally mark the bus or both simba devices not hotpluggable
> (again modelling real hardware faithfully), you can't unplug the simbas.
> I believe that's what you want.

It seems like limiting the size of the bus would solve the majority of
the problem. I've had a quick look around pci.c and while I can see that
the PCIBus creation functions take a devfn_min parameter, I can't see
anything that limits the number of slots available on the bus?

And presumably if the user did try and coldplug something into a full
bus then they would get the standard "PCI: no slot/function
available..." error?

>> My understanding from reading various bits of documentation is that the
>> the empty simba bridge (bus B) can hold a maximum of 4 devices, whilst
>> the non-empty simba bridge (bus A) can hold a maximum of 2 devices
>> (presumably due to the on-board hardware). And in order to make sure
>> OpenBIOS maps the PCI IO ranges correctly, the ebus must be the first
>> on-board device found during a PCI bus scan which means slot 0 on bus A
>> must be blacklisted.
> 
> Assuming init() plugs in the device providing ebus: plug it into slot 0,
> mark it not hotpluggable, done.

That is good solution in theory except that I'd like to keep the ebus in
slot 1 so that it matches the real DT as much as possible. In the future
it could be possible for people to boot using PROMs from a real Sun and
I'm not yet convinced that there aren't hardcoded references to some of
the onboard legacy devices in a real PROM.

>> I guess what I'm looking for is some kind of hook that runs after both
>> machine init and all the devices have been specified on the command
>> line, which I can use to validate the configuration and provide a
>> suitable error message/hint if the configuration is invalid?
> 
> You should be able to construct the machine you want, and protect the
> parts the user shouldn't mess with from messing users.  No need to
> validate the mess afterwards then.

Unfortunately there would be issues if the user was allowed to construct
a machine with more PCI devices than slots in real hardware, since the
PCI interrupt number is limited to 4 bits - 2 bits for the PCI interrupt
number (A to D), and 2 bits for the slot. So if a user tries to plug in
more than 4 devices into each simba bus then the interrupts won't be
mapped correctly.

My feeling is that it makes more sense to error out if the user tries to
add too many devices to the bus and/or in the wrong slots rather than
let them carry on and wonder why the virtual devices don't work
correctly, but I'm open to other options.


ATB,

Mark.