Re: [RFC] libxl hotplug / unplug emulated devices

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 20/05/16 17:42, Wei Liu wrote:
> On Fri, May 20, 2016 at 05:38:44PM +0100, Ian Jackson wrote:
>> Wei Liu writes ("[RFC] libxl hotplug / unplug emulated devices"):
>>> Recently I got a report on xen-users@ about xl network-attach not
>>> working for HVM guest.
>>>
>>> I try to use
>>>   xl network-attach jessie-hvm 'bridge=xenbr0'
>>> and vif-bridge script complains that it can't add vifXX-emu to bridge.
>>>
>>> The underlying issue is that the vif spec provided defaults to
>>> emulated nic, but libxl only populates a pv nic but doesn't call out
>>> via QMP to QEMU to populate one. Note that this issue not only affects
>>> nic device but essentially all device types.
>> Is it really sensible to offer emulated nic hotplug ?  That'd be
>> presented to the guest as pci hotplug, I guess ?
> Suppose you have a Windows guest doesn't have PV driver? Or any other
> OSes that have PCI drivers with hotplug support but not Xen drivers?

On qemu-trad, none of the devices support hotplug, so the option
shouldn't be available in xl.

I believe qemu-upstream does offer hotplug devices, but it still has to
create empty PCIe slots at boot time to hotplug into later, along with
appropriate ACPI tables.


>
> For the second question, yes, more or less the same if you're talking
> about libxl side implementation. It's going to call some QMP commands.
>
>>> I also experimented with block device:
>>>   xl block-attach jessie-hvm 'phy:/dev/DATA/disk,hdb,w'
>>> and it succeed, only pv disk is populated though.
>> That's what I would have expected.
>>
>> Maybe the fix should be that xl network-attach should default hotplug
>> nics to pv only.
>>
> I certainly am fine with this.

+1.

~Andrew

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