On Wed, 23 Jan 2019 09:02:34 +0100 David Hildenbrand <da...@redhat.com> wrote:
> On 22.01.19 14:01, Cornelia Huck wrote: > > On Tue, 22 Jan 2019 13:51:31 +0100 > > David Hildenbrand <da...@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. Ok, if this shows up in the config space, it is guest visible. I was thinking mostly about the zPCI enumeration, which results in a flat layout. > > 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. I'm certainly not arguing against inclusion :)
