[libvirt] [Qemu-devel] PCI(e): Documentation "io-reserve" and related properties?

[Marcel Apfelbaum]

On 6/7/19 2:43 PM, Andrea Bolognani wrote:
> On Thu, 2019-06-06 at 14:20 -0400, Michael S. Tsirkin wrote:
>> On Thu, Jun 06, 2019 at 06:19:43PM +0200, Kashyap Chamarthy wrote:
>>> Hi folks,
>>>
>>> Today I learnt about some obscure PCIe-related properties, in context of
>>> the adding PCIe root ports to a guest, namely:
>>>
>>>      io-reserve
>>>      mem-reserve
>>>      bus-reserve
>>>      pref32-reserve
>>>      pref64-reserve
>>>
>>> Unfortunately, the commit[*] that added them provided no documentation
>>> whatsover.
>>>
>>> In my scenario, I was specifically wondering about what does
>>> "io-reserve" mean, in what context to use it, etc.  (But documentation
>>> about other properties is also welcome.)
>>>
>>> Anyone more well-versed in this area care to shed some light?
>>>
>>>
>>> [*] 6755e618d0 (hw/pci: add PCI resource reserve capability to legacy
>>>      PCI bridge, 2018-08-21)
>> So normally bios would reserve just enough io space to satisfy all
>> devices behind a bridge. What if you intend to hotplug more devices?
>> These properties allow you to ask bios to reserve extra space.
> Is it fair to say that setting io-reserve=0 for a pcie-root-port
> would be a way to implement the requirements set forth in
>
>    https://bugzilla.redhat.com/show_bug.cgi?id=1408810
>
> ? I tested this on aarch64 and it seems to work as expected, but
> then again without documentation it's hard to tell.
>
> More specifically, I created an aarch64/virt guest with several
> pcie-root-ports and it couldn't boot much further than GRUB when
> the number of ports exceeded 24, but as soon as I added the
> io-reserve=0 option I could get the same guest to boot fine with
> 32 or even 64 pcie-root-ports. I'm attaching the boot log for
> reference: there are a bunch of messages about the topic but they
> would appear to be benign.
>
> Hotplug seemed to work too: I tried with a single virtio-net-pci
> and I could access the network. My understanding is that PCIe
> devices are required to work without IO space, so this behavior
> matches my expectations.
>
> I wonder, though, what would happen if I had something like
>
>    -device pcie-root-port,io-reserve=0,id=pci.1
>    -device pcie-pci-bridge,bus=pci.1
>
> Would I be able to hotplug conventional PCI devices into the
> pcie-pci-bridge, or would the lack of IO space reservation for
> the pcie-root-port cause issues with that?
>

You would not have any IO space for a PCI device or PCIe device
that for some reason will require IO space (even if they shouldn't)
and the hotplug operation would fail.

On the other hand, if the pcie-pci-bridge device itself will require
some IO space, it will work.. it worth trying.

Thanks,
Marcel