Re: [Qemu-devel] [RfC PATCH] virtio-pci: place msix regions in modern virtio bar

[Michael S. Tsirkin]

On Fri, Jan 23, 2015 at 02:53:27PM +0100, Gerd Hoffmann wrote:
>   Hi,
> 
> > I wanted to make the modern BAR prefetcheable, so it can
> > be a full 64-bit one,
> 
> It's not the case right now though, the two bars have identical
> attributes, and there is plenty of unused space in the modern virtio
> bar ...

It's just POC, I'll change it.

> > this is impossible with the MSI-X
> > BAR.
> 
> What exactly is impossible?  msi-x being in a 64bit bar?  msi-x being in
> a prefetchable bar?

This.

>  Also why the bar needs to be prefetchable to be
> 64bit bit?

It doesn't but bridges only have 32 bit windows for non prefetcheable
bars. So even if you try to make it 64 bit, only the low 32 bit are used.

> xhci looks this way right now:
> 
> 00:0f.0 USB controller: NEC Corporation uPD720200 USB 3.0 Host
> Controller (rev 03) (prog-if 30 [XHCI])
>       Subsystem: Red Hat, Inc QEMU Virtual Machine
>       Physical Slot: 15
>       Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr-
> Stepping- SERR+ FastB2B- DisINTx+
>       Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort-
> <MAbort- >SERR- <PERR- INTx-
>       Latency: 0, Cache Line Size: 64 bytes
>       Interrupt: pin A routed to IRQ 10
>       Region 0: Memory at febd0000 (64-bit, non-prefetchable) [size=16K]
>       Capabilities: [90] MSI-X: Enable+ Count=16 Masked-
>               Vector table: BAR=0 offset=00003000
>               PBA: BAR=0 offset=00003800
>       Capabilities: [70] MSI: Enable- Count=1/16 Maskable- 64bit+
>               Address: 0000000000000000  Data: 0000
>       Kernel driver in use: xhci_hcd
> 
> Which works just fine even with msi-x in a 64-bit (non-prefetchable)
> bar, but maybe just by pure luck?

xhci is integrated in a root complex so it can work there.

>  Also seabios wouldn't map the bar
> above 4G.
> 
> cheers,
>   Gerd

We can teach it to do that, and even if not, OS can do it on hotplug.