Re: Hot plug multi function PCI devices

[Laine Stump <laine@xxxxxxxxx>]

On 12/23/2015 11:01 AM, Ziviani . wrote:

Hi Laine,

This (hot plugging all functions at once) is something I was thinking about. What if we could create a xml file passing the IOMMU group instead of only one function per time, would it be feasible?

I could start working on a proof of concept if the community thinks it's a valid path.

Do you know how is currently working on it? I could offer some help if they need.

(Please reply inline rather than top-posting. It makes it much easier to follow the context of the conversation.)

What do you mean by "passing the IOMMU group"? Do you mean *just* the iommu group, excluding the information about the devices? This doesn't seem like a good idea, since afaik the iommu group number is something just conjured up by the kernel at boot time, and isn't necessarily predictable or stable between host reboots. Also, it wouldn't allow for assigning only some of the devices/functions in a group while leaving others inactive.

I think there are two reasonable possibilities:

1) Follow the apparent path of qemu - accept separate attach calls, one for each function, and use the attach of function 0 as the "action" button that causes all the functions to be attached.

2) Enhance the attach API to accept multiple <hostdev> elements in the XML for a single call, and do "whatever is proper for the current hypervisor" to attach them.


As for detach, it's really only possible to detach *all* functions, and it would take more bookkeeping to allowing marking each function for removal and then removing the device when all functions had been marked, so maybe we only allow detach of function 0, and that will always detach everything? (not sure, that's just an idea).

As far as I know, nobody is currently working on anything like this for libvirt, so this is your chance to get your hands dirty!

(It just occurred to me that method (1) of multifunction attach method outlined above will also need similar extra bookkeeping, just as the "mark each function for removal" detach method would, and this extra bookkeeping would need to survive a restart of libvirtd in the middle of a series of attach/detach calls, making it more complicated, so maybe the 2nd methods would be better. I'd love to hear opinions though.)

Thank you :)

On Mon, Dec 21, 2015 at 3:53 PM, Laine Stump <laine@xxxxxxxxx> wrote:

On 12/21/2015 08:29 AM, Ziviani . wrote:

Hello list!

I'm new here and interested in hot-plug multi-function PCI devices. Basically I'd like to know why Libvirt does not support it. I've been through the archives and basically found this thread:

https://www.redhat.com/archives/libvir-list/2011-May/msg00457.html

But Qemu seems to handle it accordingly:

virsh qemu-monitor-command --hmp fedora-23 'device_add vfio-pci,host=00:16.0,addr=08.0'

virsh qemu-monitor-command --hmp fedora-23 'device_add vfio-pci,host=00:16.3,addr=08.3'

GUEST:

# lspci

(snip)

00:08.0 Communication controller: Intel Corporation 8 Series HECI #0 (rev 04)

00:08.3 Serial controller: Intel Corporation 8 Series HECI KT (rev 04)

However, using Libvirt:

% virsh attach-device fedora-23 pci_0000_00_16_0.xml --live

Device attached successfully

% virsh attach-device fedora-23 pci_0000_00_16_3.xml --live

error: Failed to attach device from pci_0000_00_16_3.xml

error: internal error: Only PCI device addresses with function=0 are supported

I made some changes on domain_addr.c[1] for testing and it worked.

[1]https://gist.github.com/jrziviani/1da184c7fd0b413e0426

% virsh attach-device fedora-23 pci_0000_00_16_3.xml --live

Device attached successfully

GUEST:

# lspci

(snip)

00:08.0 Communication controller: Intel Corporation 8 Series HECI #0 (rev 04)

00:08.3 Serial controller: Intel Corporation 8 Series HECI KT (rev 04)

So there is more to it that I'm not aware?

You're relying on behavior in the guest OS for which there is no standard (and which, by definition, doesn't work on real hardware, so no guest OS will be expecting it; a friend more familiar with this has told me that probably qemu is sending an (acpi?) "device check" to the guest for each function that is added, and in your case it's apparently "doing the right thing" in response to that). But just because it is successful in this one case doesn't mean that it will be successful in all situations; likely it won't be. So while the qemu monitor takes the laissez-faire approach of allowing you to try it and letting you pick up the pieces when it fails, libvirt prevents it because it is bound to fail, and thus not supportable.

There has recently been some work in qemu to "save up" any requests to attach devices with function > 0, then present them all to the guest at once when function 0 is attached. This is the only standard way to handle hotplug of multiple functions in a slot. Hot unplug can only happen for all functions in the slot at once. I'm not sure of the current status of that work, but once it is in and stable, libvirt will support it.

Thank you!

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