Re: Xen 4.2 and PCI hotplug.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Wed, Dec 19, 2012 at 03:08:31PM +0000, Jan Beulich wrote:
> >>> On 19.12.12 at 15:36, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
> > On 19/12/2012 09:14, Zhang, Xiantao wrote:
> >>> -----Original Message-----
> >>> From: Jan Beulich [mailto:JBeulich@suse.com]
> >>> Sent: Wednesday, December 19, 2012 4:48 PM
> >>> To: Zhang, Xiantao
> >>> Cc: Konrad Rzeszutek Wilk; xen-devel
> >>> Subject: Re: [Xen-devel] Xen 4.2 and PCI hotplug.
> >>>
> >>>>>> On 19.12.12 at 09:13, "Zhang, Xiantao" <xiantao.zhang@intel.com>
> >>> wrote:
> >>>> Are you playing with Xen ?  so far,  Xen doesn't support PCIe device
> >>>> hot-plug feature yet.
> >>> When saying Xen, I assume you mean the pv-ops kernel instead? So far I was
> >>> under the impression that this worked even with the very old 2.6.18 tree (as
> >>> much or as little as hotplug there worked in the native case). And given 
> > that
> >>> there are no special requirements on the hypervisor to make this work, it's
> >>> not even obvious to me what would be missing in the pv-ops kernel to make
> >>> it work. 
> >> Oh, my fault!  Perhaps we don't need to do anything for pv-ops kernel to 
> > support device hot-plug if native system has it supported.  Actually, we 
> > didn't do such testings before, since it is a native feature, not a 
> > Xen-specific one. 
> >> Xiantao
> > 
> > My current understanding is that on boot, Xen scans the PCI bus, then
> > dom0 rescans it later.  If a hotplug event gets serviced by dom0, does
> > there not need to be some hypercall informing Xen that a new device has
> > appeared?  I expect PCIPassthrough would not work correctly on a
> > hotplugged device which Xen is unaware of.
> 
> Sure - such a hypercall exists and is - from all I can tell - being made
> not only during the boot time bus scan, but also during hotplug
> processing. See drivers/xen/pci.c.

Right. What I am not sure about - and this is why I am asking Intel - whether
that notifier gets called _after_ dom0 has interogated the new PCI device.

And if so, would Xen (which gets trapped on the 0xcf8) would not let dom0
actually interogate the new BDF b/c it hasn't been told about this BDF
existence.

This is all speculation - and I might be very well completly wrong about
all of this - hence this email-thread.

Testing would of course tell for sure, but I am not sure if I even have
such hardware.
> 
> Jan
> 
> 
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