Re: [Qemu-devel] [PATCH v1 0/2] s390x/pci: PCI bridge plugging fixes

[David Hildenbrand <david@redhat.com>]

On 22.01.19 14:01, Cornelia Huck wrote:
> On Tue, 22 Jan 2019 13:51:31 +0100
> David Hildenbrand <david@redhat.com> wrote:
> 
>> Hotplugging of PCI bridges is right now pretty much broken. Coldplugging
>> and hotplugging will assign wrong primary bus numbers in some scenarios.
>>
>> I base my knowledge on how this is supposed to work on
>> http://www.science.unitn.it/~fiorella/guidelinux/tlk/node76.html
>>
>> I did a couple of tests, building whole hierarchies of bridges, both
>> hot and coldplugged. "info pci" as well as the Linux guests showed
>> what I was expecting.
>>
>> David Hildenbrand (2):
>>   s390x/pci: Fix primary bus number for PCI bridges
>>   s390x/pci: Fix hotplugging of PCI bridges
>>
>>  hw/s390x/s390-pci-bus.c | 41 +++++++++++++++++++++++++++--------------
>>  1 file changed, 27 insertions(+), 14 deletions(-)
>>
> 
> I'll leave the actual review of this to folks familiar with how zPCI is
> supposed to work :)
> 
> Does the guest actually see anything of this? Consistency is good, and
> not crashing even better, but I think all of the topology is invisible
> on the guest side anyway...
> 

I am no PCI expert, but I think the guest is able to read/write these
numbers via the configuration space. As far as I can see, on x86 the
BIOS builds the topology. On Power/s390x this job is delegated to
firmware / QEMU.

There are quite some numbers in QEMU relying on these numbers to be
correct: E.g. pci_secondary_bus_in_range()

If the guest relies on the topology to be correct, things can go wrong.
I have no idea how Linux guests actually use the topology.

Also, the output of "info pci" will be wrong.

Anyhow, this is the right thing to do, but I agree that Patch #1 might
not be as critical as patch #2.

-- 

Thanks,

David / dhildenb