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

Anthony Liguori [off-list ref] writes:

Gerd Hoffmann [off-list ref] writes:

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  Hi,

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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.

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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.

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.