Re: [Xen-devel] [PATCH 3/4] Allow vif= to specify PCI address for each nic

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Jun 15, 2015 at 01:45:26PM -0400, Don Slutz wrote:
> On 06/15/15 11:54, Wei Liu wrote:
> > On Mon, Jun 15, 2015 at 10:15:51AM -0400, Don Slutz wrote:
> >> This allows more then 32 nics.
> >>
> > 
> > How does this patch help? What prevents you from having more than 32
> > nics?
> > 
> 
> Without a way to put the emulated nics onto a PCI to PCI Bridge, you are
> limited to 1 PCI bus (the host one).
> 
> A PCI bus has a max of 32 PCI devices.
> 
> So, since the host bus has some PCI devices that are not nics, the limit
> is smaller then 32.
> 

Is there anything that you can't accomplish by using
device_model_args_hvm?

> 
> >> This can help with Windows finding nics at boot time.
> >>
> >> This allows changing config file:
> >>
> >>     builder = "hvm"
> >>     device_model_args_hvm = [
> >>      "-device",
> >>      
> >> "pci-bridge,chassis_nr=2,msi=on,id=pciBridge5.0,multifunction=on,addr=0x15.0",
> >>      "-device",
> >>      
> >> "vmxnet3,id=nic3,netdev=net3,mac=00:0c:29:86:44:be,bus=pciBridge5.0,addr=0x4.0x0",
> >>      "-netdev",
> >>      
> >> "type=tap,id=net3,ifname=vif.3-emu,script=/etc/qemu-ifup,downscript=no",
> >>     ]
> >>     vif = [
> >>     ]
> >>
> >> to:
> >>
> >>     builder = "hvm"
> >>     device_model_args_hvm = [
> >>      "-device",
> >>      
> >> "pci-bridge,chassis_nr=2,msi=on,id=pciBridge5.0,multifunction=on,addr=0x15.0",
> >>     ]
> >>     vif = [
> >>      
> >> "model=vmxnet3,bridge=xenbr0,mac=00:0c:29:86:44:a0,bus=pciBridge5.0,addr=0x4.0x0",
> >>     ]
> >>

The way that you construct PCI bridge via device_model_args_hvm while
plumbing some other information via vif= is fragile and not general
useful to others. I don't think I would accept a half-baked solution
like this.

> >> which enables usage of xen-netback.
> >>
> > 
> > In any case, exposing HVM-only options to top-level vif configuration
> > space doesn't look right.
> 
> 
> There are already HVM-only options in vifs:
> 
> ### type
> 
> This keyword is valid for HVM guests only.
> ...
> ### model
> 
> This keyword is valid for HVM guest devices with `type=ioemu` only.
> ...
> 
> > Why do you want to set bus and addr? The
> > rationale should be stated in commit message.
> 
> 
> That is why I said:
> 
> >> This can help with Windows finding nics at boot time.
> 
> Windows boot code is not as flexible as Linux.  Most versions of Windows
> like to blue screen if the hardware changes enough.
> 

Looks like you're trying to migrate a guest from VMWare to Xen. If
device_model_args_new is sufficient please just use that.

Wei.

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