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

[Heyi Guo]

Hi Eric,

Could you help to confirm?

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
> >
> >
> > .