Re: [PATCH 4/4] vl: Prioritize realizations of devices

[Michael S. Tsirkin]

On Mon, Aug 23, 2021 at 05:31:46PM -0400, Peter Xu wrote:
> On Mon, Aug 23, 2021 at 05:07:03PM -0400, Eduardo Habkost wrote:
> > To give just one example:
> > 
> > $ (echo 'info pci';echo quit;) | qemu-system-x86_64 -device virtio-net-pci 
> > -device e1000e -monitor stdio | tail -n 20
> >   Bus  0, device   4, function 0:
> >     Ethernet controller: PCI device 1af4:1000
> >       PCI subsystem 1af4:0001
> >       IRQ 0, pin A
> >       BAR0: I/O at 0xffffffffffffffff [0x001e].
> >       BAR1: 32 bit memory at 0xffffffffffffffff [0x00000ffe].
> >       BAR4: 64 bit prefetchable memory at 0xffffffffffffffff [0x00003ffe].
> >       BAR6: 32 bit memory at 0xffffffffffffffff [0x0003fffe].
> >       id ""
> >   Bus  0, device   5, function 0:
> >     Ethernet controller: PCI device 8086:10d3
> >       PCI subsystem 8086:0000
> >       IRQ 0, pin A
> >       BAR0: 32 bit memory at 0xffffffffffffffff [0x0001fffe].
> >       BAR1: 32 bit memory at 0xffffffffffffffff [0x0001fffe].
> >       BAR2: I/O at 0xffffffffffffffff [0x001e].
> >       BAR3: 32 bit memory at 0xffffffffffffffff [0x00003ffe].
> >       BAR6: 32 bit memory at 0xffffffffffffffff [0x0003fffe].
> >       id ""
> > (qemu) quit
> > $ (echo 'info pci';echo quit;) | qemu-system-x86_64 -device e1000e -device 
> > virtio-net-pci -monitor stdio | tail -n 20
> >   Bus  0, device   4, function 0:
> >     Ethernet controller: PCI device 8086:10d3
> >       PCI subsystem 8086:0000
> >       IRQ 0, pin A
> >       BAR0: 32 bit memory at 0xffffffffffffffff [0x0001fffe].
> >       BAR1: 32 bit memory at 0xffffffffffffffff [0x0001fffe].
> >       BAR2: I/O at 0xffffffffffffffff [0x001e].
> >       BAR3: 32 bit memory at 0xffffffffffffffff [0x00003ffe].
> >       BAR6: 32 bit memory at 0xffffffffffffffff [0x0003fffe].
> >       id ""
> >   Bus  0, device   5, function 0:
> >     Ethernet controller: PCI device 1af4:1000
> >       PCI subsystem 1af4:0001
> >       IRQ 0, pin A
> >       BAR0: I/O at 0xffffffffffffffff [0x001e].
> >       BAR1: 32 bit memory at 0xffffffffffffffff [0x00000ffe].
> >       BAR4: 64 bit prefetchable memory at 0xffffffffffffffff [0x00003ffe].
> >       BAR6: 32 bit memory at 0xffffffffffffffff [0x0003fffe].
> >       id ""
> > (qemu) quit
> > 
> > 
> > If the order of the -device arguments changes, the devices are assigned to
> > different PCI slots.
> 
> Thanks for the example.
> 
> Initially I thought about this and didn't think it an issue (because serious
> users will always specify addr=XXX for -device; I thought libvirt always does
> that), but I do remember that guest OS could identify its hardware config with
> devfn number, so nmcli may mess up its config with before/after this change
> indeed..
> 
> I can use a custom sort to replace qsort() to guarantee that.


You don't have to do that. Simply use the device position on the command
line for comparisons when priority is the same.


> Do you have other examples in mind that I may have overlooked, especially I 
> may
> not be able to fix by a custom sort with only moving priority>=1 devices?
> 
> Thanks,

> -- 
> Peter Xu