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
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