Re: [Xen-devel] PCI hotplug related issues

[Anil Rao <anil.rao@xxxxxxxxxxxx>]

Thanks for the reply. 

After encountering this problem, I was afraid that only VM-level hotplug is 
supported by Xen (and not true hotplug at the host-level). Can someone kindly 
confirm that this is the case? Our current effort depends on host-level hotplug 
of a PCI device that can (later) be assigned (hotplugged) to a running VM, so a 
definitive answer will help us to avoid going down the wrong path.

At the same time, I still think its odd that the Xen xm pci-attach command 
returned success when the attempted hotplug operation actually failed (as 
reported by the error message printed in the qemu-dm log file). Also, as I have 
indicated in my earlier post, Xen actually believes that the PCI device is 
assigned to the target VM (verified this by using xm pci-list to find out the 
list of devices attached to a VM), which again is very misleading. 

-Anil



-----Original Message-----
From: Jan Beulich [mailto:JBeulich@xxxxxxxx] 
Sent: Tuesday, June 05, 2012 1:43 AM
To: Anil Rao
Cc: xen-devel
Subject: Re: [Xen-devel] PCI hotplug related issues

>>> On 04.06.12 at 21:00, Anil Rao <anil.rao@xxxxxxxxxxxx> wrote:
> A) Does Xen support case 1 (hotplugging a PCI device to a running VM 
> that was powered-on before the device was added to
>     the host)?

As far as I'm aware, only hotplug from the perspective of the guest is really 
supported/tested.

But the issue here really seems less Xen related, but more towards libpci 
behavior (as according to qemu this is where the device isn't known). As that's 
a component Xen/qemu-dm simply builds upon, failure there (e.g. lack of hotplug 
awareness) would surface as a Xen misbehavior. Quickly going through the list 
of provided symbols of libpci.so doesn't show anything that looks hotplug 
related (e.g.
some sort of notification registration). But of course it's possible that I'm 
overlooking something here, and that in fact qemu-dm's interaction with libpci 
is simply incomplete.

Jan


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