Re: [Qemu-devel] [PATCH V7 23/24] apci: fix PXB behaviour if used with unsupported BIOS

[Laszlo Ersek]

On 06/02/15 13:37, Marcel Apfelbaum wrote:
> On 06/01/2015 06:37 PM, Laszlo Ersek wrote:
>> On 06/01/15 15:48, Marcel Apfelbaum wrote:
>>> On 06/01/2015 04:28 PM, Laszlo Ersek wrote:
>>>> On 06/01/15 15:05, Marcel Apfelbaum wrote:
>>>>> On 06/01/2015 03:27 PM, Michael S. Tsirkin wrote:
>>>>>> On Mon, Jun 01, 2015 at 03:21:19PM +0300, Marcel Apfelbaum wrote:
>>>>>>> On 06/01/2015 03:17 PM, Michael S. Tsirkin wrote:
>>>>>>>> On Mon, Jun 01, 2015 at 01:40:19PM +0200, Gerd Hoffmann wrote:
>>>>>>>>> On Mo, 2015-06-01 at 12:44 +0300, Marcel Apfelbaum wrote:
>>>>>>>>>> On 05/31/2015 09:12 PM, Michael S. Tsirkin wrote:
>>>>>>>>>>> On Mon, May 25, 2015 at 06:34:01PM +0300, Marcel Apfelbaum
>>>>>>>>>>> wrote:
>>>>>>>>>>>> PXB does not work with unsupported bioses, but should
>>>>>>>>>>>> not interfere with normal OS operation.
>>>>>>>>>>>> We don't ship them anymore, but it's reasonable
>>>>>>>>>>>> to keep the work-around until we update the bios in qemu.
>>>>>>>>>>>
>>>>>>>>>>> We already did, did we not?
>>>>>>>>>> Yes, we did, but Gerd preferred to keep this patch around.
>>>>>>>>>> Adding him to thread.
>>>>>>>>>
>>>>>>>>> seabios bundled with qemu isn't the only possible firmware.
>>>>>>>>>
>>>>>>>>> We have ovmf, coreboot, qboot.
>>>>>>>>
>>>>>>>> ovmf is especially interesting. Marcel, did you look at what
>>>>>>>> happens with pxb and ovmf?
>>>>>>> No, I talked to Laszlo about it, he said ovmf is not there yet.
>>>>>>> OVMF will not query the extra buses, so the devices on the extra bus
>>>>>>> will not be visible.
>>>>>>> Adding him to the thread.
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Marcel
>>>>>>
>>>>>> But does OVMF need this specific patch?
>>>>> I don't think so because more than likely it doesn't scan for the
>>>>> extra
>>>>> buses,
>>>>> so it will not try to configure these devices.
>>>>> Laszlo, am I right?
>>>>
>>>> Well, I don't know. :)
>>>>
>>>> First, I'm not seeing the specific patch in question (can you pls send
>>>> me a URL into the web archive, or a Message-Id?)
>>> Well, there are a few patches, all this series,
>>> You can look for patches:
>>> 13/24 hw/acpi: add support for i440fx 'snooping' root busses -> acpi
>>> declarations
>>> 18/24 hw/pci: introduce PCI Expander Bridge (PXB)
>>> 19/24 hw/pci: inform bios if the system has extra pci root buses
>>>
>>> Basically we add the pxb resources to ACPI tables and then inform BIOS
>>> using
>>> etc/extra-pci-roots fw_config file that he has extra roots to scan.
>>>
>>> If the OVMF only looks for bus 0 and does not scan all possible buses
>>> it will not see PXB's root bus
>>
>> I don't know enough about PCI to reply sensibly.
>>
>> I can tell you that the bus range in OVMF, from "mResAperture" in
>> "PcAtChipsetPkg/PciHostBridgeDxe/PciHostBridge.c", is [0..0xff],
>> inclusive.
>>
>> This bus range is then exposed in the function StartBusEnumeration() to
>> the caller (which is the PCI bus driver), as an output parameter. The
>> StartBusEnumeration() function has the following leading comment:
>>
>>> Sets up the specified PCI root bridge for the bus enumeration process.
>>>
>>> This member function sets up the root bridge for bus enumeration and
>>> returns the PCI bus range over which the search should be performed
>>> in ACPI 2.0 resource descriptor format.
>>
>> So, there's a chance that if those busses actually exist on the virtual
>> hardware, the drivers included by OVMF from the generic edk2 source
>> "will just work". It is also possible that OVMF will notice no change at
>> all.
>>
>> Do you have a public branch, and a matching command line? The PCI
>> enumeration / resource allocation spews a bunch of messages in OVMF, so
>> if you placed (on the QEMU command line) some devices on one of these
>> nonzero buses, then their enumeration / resource allocation, determined
>> from the log, could serve as evidence. (I think this should be testable
>> on a non-NUMA host, and without passthrough devices as well.)
>>
>> Also, I checked the actual code hunks & message body for this patch (ie.
>> 23/24) on the web. Looks like I should be able to dump the ACPI tables
>> in the guest, and get those dumps to you for verification. OVMF does
>> delay the ACPI download until after PCI enumeration, so the state of the
>> guest _CRS would be (negative or positive) proof, not lack of proof.
> 
> Hi Laszlo,
> I am sorry for the late reply.
> Can you please check using mst branch?
>     git://git.kernel.org/pub/scm/virt/kvm/mst/qemu.git pxb
> Just add to the regular command line:
>     -device pxb,id=bridge1,bus_nr=4 -netdev user,id=u -device
> e1000,id=net2,bus=bridge1,netdev=u,addr=1
> 
> Thanks a lot for the help, we mainly want to know if there is
> an architecture issue that will prevent the PXB to work with OVMF.
> Marcel

I wrote a horrible patch that allowed OVMF to enumerate the e1000 NIC on
the pxb, so I guess there should be no "architectural issue" preventing
OVMF from using this device. Of course, making good / dynamic use of
this stuff is light years away. I'll respond with more details in the
thread you started on edk2-devel:

http://thread.gmane.org/gmane.comp.bios.tianocore.devel/15147

Thanks
Laszlo


>>>
>>>> Second, recently I tested OVMF on Q35, but not just with a simple /
>>>> usual command line invocation -- I tested it on a Q35 machine
>>>> configured
>>>> by libvirt. That's a very different animal.
>>>>
>>>> While it exposed a problem in OVMF's own boot order processing:
>>>>
>>>> https://github.com/tianocore/edk2/commit/feca17fa4b
>>>>
>>>> I was surprised to see that the PCI bus driver enumerated devices
>>>> behind
>>>> two bridges no less without any problems. So, bridges off the one root
>>>> bridge should work, but several root bridges probably won't. (Exposing
>>>> root bridges is the responsibility of another driver, and they are not
>>>> enumerable in the usual way.)
>>>>
>>>> Thanks
>>>> Laszlo
>>>>
>>>
>>
>