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Re: [Qemu-devel] [PATCH v1 1/2] s390x/pci: Fix primary bus number for PC

From: Thomas Huth
Subject: Re: [Qemu-devel] [PATCH v1 1/2] s390x/pci: Fix primary bus number for PCI bridges
Date: Wed, 23 Jan 2019 11:26:20 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 2019-01-22 13:51, David Hildenbrand wrote:
> The primary bus number corresponds always to the bus number of the
> bus the bridge is attached to.
> Right now, if we have two bridges attached to the same bus (e.g. root
> bus) this is however not the case. Fix assignment.
> While at it
> - Add a comment why we have to reassign durign every reset (which I


>   found to be supprising)


> - Drop setting the PCI_SUBORDINATE_BUS temporarily to 0xff. As we are
>   setting it via a DFS and not via a BFS (as discussed e.g. in [1]), this
>   is not necessary. The last number when we return is the highest
>   number.

I think that explanation is slightly wrong / misleading. It's not about
DFS vs. BFS, it's about guest code vs. QEMU code. If you do a bridge
setup from the guest side, you've got to set the subordinate bus number
to 0xff to make sure that the bridge forwards all config space accesses
to the attached devices while you're scanning the devices that are
attached to the bridge.
But this code is not running in the guest, it is running in QEMU, and so
it can access the config space of the attached devices directly via
pci_default_write_config() - the write do not need to pass the parent
bridge in this case, and thus the subordinate bus number in the bridge
is ignored for the config space write access.

> Please note that hotplugging of bridges is in general still broken, will
> be fixed next.
> [1] http://www.science.unitn.it/~fiorella/guidelinux/tlk/node76.html
> Signed-off-by: David Hildenbrand <address@hidden>
> ---
>  hw/s390x/s390-pci-bus.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)

With the commit message fixed:

Reviewed-by: Thomas Huth <address@hidden>

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