Re: [Qemu-devel] Using PCI config space to indicate config location

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Oct 09, 2012 at 10:26:12AM +1030, Rusty Russell wrote:
> Anthony Liguori <aliguori@us.ibm.com> writes:
> > Gerd Hoffmann <kraxel@redhat.com> writes:
> >
> >>   Hi,
> >>
> >>>> So we could have for virtio something like this:
> >>>>
> >>>>         Capabilities: [??] virtio-regs:
> >>>>                 legacy: BAR=0 offset=0
> >>>>                 virtio-pci: BAR=1 offset=1000
> >>>>                 virtio-cfg: BAR=1 offset=1800
> >>> 
> >>> This would be a vendor specific PCI capability so lspci wouldn't
> >>> automatically know how to parse it.
> >>
> >> Sure, would need a patch to actually parse+print the cap,
> >> /me was just trying to make my point clear in a simple way.
> >>
> >>>>>> 2) ISTR an argument about mapping the ISR register separately, for
> >>>>>>    performance, but I can't find a reference to it.
> >>>>>
> >>>>> I think the rationale is that ISR really needs to be PIO but everything
> >>>>> else doesn't.  PIO is much faster on x86 because it doesn't require
> >>>>> walking page tables or instruction emulation to handle the exit.
> >>>>
> >>>> Is this still a pressing issue?  With MSI-X enabled ISR isn't needed,
> >>>> correct?  Which would imply that pretty much only old guests without
> >>>> MSI-X support need this, and we don't need to worry that much when
> >>>> designing something new ...
> >>> 
> >>> It wasn't that long ago that MSI-X wasn't supported..  I think we should
> >>> continue to keep ISR as PIO as it is a fast path.
> >>
> >> No problem if we allow to have both legacy layout and new layout at the
> >> same time.  Guests can continue to use ISR @ BAR0 in PIO space for
> >> existing virtio devices, even in case they want use mmio for other
> >> registers -> all fine.
> >>
> >> New virtio devices can support MSI-X from day one and decide to not
> >> expose a legacy layout PIO bar.
> >
> > I think having BAR1 be an MMIO mirror of the registers + a BAR2 for
> > virtio configuration space is probably not that bad of a solution.
> 
> Well, we also want to clean up the registers, so how about:
> 
> BAR0: legacy, as is.  If you access this, don't use the others.
> BAR1: new format virtio-pci layout.  If you use this, don't use BAR0.
> BAR2: virtio-cfg.  If you use this, don't use BAR0.
> BAR3: ISR. If you use this, don't use BAR0.

One problem here is there are only 3 64-bit BARs under PCI.
There's no point to make IO BAR 64-bit on x86, but there might
be for memory.

> I prefer the cases exclusive (ie. use one or the other) as a clear path
> to remove the legacy layout; and leaving the ISR in BAR0 leaves us with
> an ugly corner case in future (ISR is BAR0 + 19?  WTF?).
> 
> As to MMIO vs PIO, the BARs are self-describing, so we should explicitly
> endorse that and leave it to the devices.
> 
> The detection is simple: if BAR1 has non-zero length, it's new-style,
> otherwise legacy.
> 
> Thoughts?
> Rusty.