Re: [Qemu-devel] [PATCH] hw/pci: do not update the PCI mappings while Decode (I/O or memory) bit is not set in the Command register

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jan 14, 2016 at 07:20:32PM +0200, Marcel Apfelbaum wrote:
> On 01/14/2016 05:37 PM, Michael S. Tsirkin wrote:
> >On Thu, Jan 14, 2016 at 05:23:27PM +0200, Marcel Apfelbaum wrote:
> >>On 01/14/2016 04:49 PM, Michael S. Tsirkin wrote:
> >>>On Thu, Jan 14, 2016 at 03:30:41PM +0100, Laszlo Ersek wrote:
> >>>>>2. The same as with pxb, disable Integrated End points for pxb-pcie.
> >>>>
> >>>>My vote, without a doubt.
> >>>
> >>>Yea, me too.
> >>>
> >>>
> >>>On a related note: I wonder whether enough resources will be allocated
> >>>to the bridge to actually make it possible to add devices by hotplug
> >>>later.
> >>>
> >>
> >>It works the same as with PXB, but now instead of having one internal PCI-bridge,
> >>we will have several switches/root ports. Each of them will get the minimum MEM required by
> >>PCI bridges,
> >
> >what does this mean? What if you add a bunch of devices
> >with large memory BARs? They won't fit will they?
> >
> 
> Indeed, devices with over 1 MB (I think) BARs can't be hot-plugged.
> This is a known design limitation. We can think of a way to handle this,
> but the real reason we have multiple root bridges is to be able to
> correlate an assigned device with a NUMA node. In this case the device
> will be added more likely at boot time.

Ugh. That's pretty nasty, esp considering live
migration pretty much requires hotplug ATM.

> 
> I think the first step is to have *some* hot-plug support for pxb/pxb-pcie
> with the current constraints, once it works we can think
> of a way to make it work for devices with large BARs.
> 
> Thanks,
> Marcel

Well OK but I suspect changes will require host/guest interface changes.
Time enough before 2.6 but I would hate to release 2.6 with this
limitation in place.

And I'd like to mention a real pci express host won't
have this issue I think as it is normally allocated
a range of memory at boot time.


> >>however the IO will be allocated only if at least one legacy device
> >>will be present at boot time. (this is at least what SeaBIOS does, I am going to check OVMF actions)
> >>
> >>Also related, checking that PCIe native hotplug works for devices behind
> >>pxb-pcie bridges is my next step after I fix the current issue.
> >>
> >>Thanks,
> >>Marcel
> >>
> >>>
> >>>>>
> >>>>>I am going to look at 1., maybe I is doable in a clean way.
> >>>>
> >>>>My vote: don't. :)
> >>>>
> >>>>Thanks
> >>>>Laszlo
> >>>>
> >>>>>Thanks,
> >>>>>Marcel
> >>>>>
> >>>>>
> >>>>>[...]