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


From: Marcel Apfelbaum
Subject: Re: [Qemu-devel] PCI(e): Documentation "io-reserve" and related properties?
Date: Tue, 11 Jun 2019 13:15:16 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1



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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