[libvirt] [PATCH v2 7/8] Move the detach of PCI device to the beginnging of live hotplug

Laine Stump laine at laine.org
Thu May 19 19:06:48 UTC 2016

On 05/19/2016 02:46 PM, Shivaprasad bhat wrote:
> On Fri, May 20, 2016 at 12:05 AM, Laine Stump <laine at laine.org 
> <mailto:laine at laine.org>> wrote:
>     On 05/18/2016 05:35 PM, Shivaprasad G Bhat wrote:
>         The hostdevices are the only devices which have dependencies
>         outside of themselves such that, other functions of the PCI
>         card should also have been detached from host driver before
>         attempting the hotplug.
>     Are you saying that all the functions on a host device must be
>     detached from their host driver, even if you're only assigning one
>     or another of them to the guest?
> Yes. All devices from same iommu group should be detached from the host.

Ah right, I forgot about that. But that is something we already don't 
deal with (currently you can assign a single device from an iommu group 
with many devices, but managed='yes' won't work for it. That would 
require some other sort of managed mode, like "managed='group'" or 
something, but more elaborate managed modes have already been NACKed on 
the list (I posted an RFC patch for "managed='detach'" at the request of 
someone else - it would detach the device from the host driver before 
assignment, but not reattach it afterwards. This was deemed 
inappropriate and unnecessary.)

You may wonder why we don't want to automatically detach all the other 
devices without any clue to do so in the config - it's because you can't 
expect the user to know which devices are in the same iommu group as the 
one you want to assign, and you don't want to just automatically/silent 
detach one that is essential for continued operation of the host.

So we really should only detach those devices that are going to be 
assigned to the guest.

> Here, I hope the Card is independent. Otherwise, many cards can also
> belong to same iommu group. In such case, manual the nodedev-detach 
> for other card functions is necessary.
>         This patch moves the detach to the beginning of the hotplug
>         so that the following patch can detach all funtions first before
>         attempting to hotplug any.
>         We need not move the detach for net devices using SRIOV as
>         all SRIOV devices are single function devices.
>     I'm not sure why that makes any difference. In any case, you
>     should detach all the devices that are going to be assigned, then
>     assign them all, with the one going to function 0 being last.
> There will be only function zero. So I felt its not necessary. Are you 
> saying different SRIOV functions(all with function zero) be hotplugged as
> different functions of single card in guest ? In that case, we would 
> need to do that.

I couldn't parse your question, but I wasn't talking about SRIOV in 
particular, just saying that the way multifunction device assignment is 
done in qemu is to assign all the non-0 functions, then assign function 
0. But that was just an incidental part of the conversation. The 
important part is that you first detach from the host all devices to be 
assigned, then you assign them all.

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