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

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?