Using PCI config space to indicate config location

[Rusty Russell <rusty@rustcorp.com.au>]

(Topic updated, cc's trimmed).

Anthony Liguori <aliguori@us.ibm.com> writes:
> Rusty Russell <rusty@rustcorp.com.au> writes:
>> 4) The only significant change to the spec is that we use PCI
>>    capabilities, so we can have infinite feature bits.
>>    (see http://lists.linuxfoundation.org/pipermail/virtualization/2011-December/019198.html)
>
> We discussed this on IRC last night.  I don't think PCI capabilites are
> a good mechanism to use...
>
> PCI capabilities are there to organize how the PCI config space is
> allocated to allow vendor extensions to co-exist with future PCI
> extensions.
>
> But we've never used the PCI config space within virtio-pci.  We do
> everything in BAR0.  I don't think there's any real advantage of using
> the config space vs. a BAR for virtio-pci.

Note before anyone gets confused; we were talking about using the PCI
config space to indicate what BAR(s) the virtio stuff is in.  An
alternative would be to simply specify a new layout format in BAR1.

The arguments for a more flexible format that I know of:

1) virtio-pci has already extended the pci-specific part of the
   configuration once (for MSI-X), so I don't want to assume it won't
   happen again.

2) ISTR an argument about mapping the ISR register separately, for
   performance, but I can't find a reference to it.

> This maps really nicely to non-PCI transports too.

This isn't right.  Noone else can use the PCI layout.  While parts are
common, other parts are pci-specific (MSI-X and ISR for example), and
yet other parts are specified by PCI elsewhere (eg interrupt numbers).

> But extending the
> PCI config space (especially dealing with capability allocation) is
> pretty gnarly and there isn't an obvious equivalent outside of PCI.

That's OK, because general changes should be done with feature bits, and
the others all have an infinite number.  Being the first, virtio-pci has
some unique limitations we'd like to fix.

> There are very devices that we emulate today that make use of extended
> PCI device registers outside the platform devices (that have no BARs).

This sentence confused me?

Thanks,
Rusty.