Re: [Qemu-devel] Question: can we hot plug a PCIe switch on machine "virt"

[Heyi Guo]

Hi Eric and Michael,

Appreciate your sharing of this information. But sorry I'm still a little 
confused due to my poor English...


On 2019/4/11 20:19, Auger Eric wrote:
> Hi Heyi,
>
> On 4/11/19 1:30 PM, Heyi Guo wrote:
> > Hi Eric,
> >
> > Could you help to confirm?
> Practically I have not tried anything else than hot-plugging the
> downstream port downstream to the upstream port. This looks not
> supported as documented.
Do you mean you *have* tried hot-plugging the downstream port to the upstream 
port? Have you succeeded? Could you tell how to do that?

> Michael answered that up to him, the downstream port could only be
> plugged downto the upstream port.
Is only code-plugging supported here, or hot-plugging is supported as well?

Thanks,
Heyi

> On my side I have not aware of any usage of the downstream port outside
> of the switch's scope.
>
> Thanks
>
> Eric
> > Thanks,
> >
> > Heyi
> >
> >
> > On 2019/4/7 9:59, Heyi Guo wrote:
> > > Hi Eric,
> > >
> > > My real interesting is about the hotplug of PCIe switch, which means
> > > we don't need to provide lots of PCIe root ports or PCIe down stream
> > > ports at the beginning, but we can extend the capacity by hot adding
> > > PCIe switches which can provide more hot-pluggable slots for endpoint
> > > devices.
> > >
> > > The document docs/pcie.txt says "PCI Express Downstream Ports can't be
> > > hot-plugged into an existing PCI Express Upstream Port" which confuses
> > > me. Does it actually mean Downstream Ports can't be hot-plugged? For
> > > they can't be hot-plugged into an existing Upstream Port as the doc
> > > says, either they can't be hot-plugged into an non-existing Upstream
> > > Port or another place...
> > >
> > > Thanks,
> > >
> > > Heyi
> > >
> > >
> > > On 2019/4/4 15:39, Auger Eric wrote:
> > > > Hi Heyi,
> > > >
> > > > On 4/3/19 8:50 PM, Michael S. Tsirkin wrote:
> > > > > On Wed, Apr 03, 2019 at 03:32:09PM +0800, Heyi Guo wrote:
> > > > > > Hi folks,
> > > > > >
> > > > > > In physical world, a PCIe switch including one upstream port and
> > > > > > several downstream ports is a single physical device, however we
> > > > > > treat each port as a device in qemu world. In qemu docs/pcie.txt,
> > > > > > we have below statements:
> > > > > >
> > > > > > Line 230: Be aware that PCI Express Downstream Ports can't be
> > > > > > hot-plugged into
> > > > > > Line 231: an existing PCI Express Upstream Port.
> > > > > >
> > > > > > To my understanding, it implies PCIe downstream ports *can* be
> > > > > > hot-plugged into something which is not an existing upstream port.
> > > > > > If it is true, how can we do that? AFAIK monitor command device_add
> > > > > > can only add one device at a time.
> > > > > >
> > > > > > Please help to show the truth.
> > > > > >
> > > > > > Thanks,
> > > > > >
> > > > > > Heyi
> > > > > afaik they can only be plugged into upstearm ports with or without
> > > > > hotplug.
> > > > Hotplug on upstream port does not look supported, as mentionned in
> > > > the doc:
> > > > (QEMU) device_add driver=xio3130-downstream id=down0 bus=upstream_port1
> > > > {"error": {"class": "GenericError", "desc": "Bus 'upstream_port1' does
> > > > not support hotplugging"}}
> > > >
> > > > Looks the std way to use the downstream port is the one documented in
> > > > 2.2.3:
> > > >
> > > > 2.2.3 Plugging a PCI Express device into a Switch:
> > > > -device ioh3420,id=root_port1,chassis=x,slot=y[,bus=pcie.0][,addr=z]
> > > > -device x3130-upstream,id=upstream_port1,bus=root_port1[,addr=x]
> > > > -device
> > > > xio3130-downstream,id=downstream_port1,bus=upstream_port1,chassis=x1,slot=y1[,addr=z1]]
> > > >
> > > > -device <dev>,bus=downstream_port1
> > > >
> > > > For my curiosity why do you want to hotplug a downstream port in another
> > > > place than an upstream port?
> > > >
> > > > Thanks
> > > >
> > > > Eric
> > > >
> > > >
> > > > .
> .