Re: [RFC PATCHv2] x86/pci: Initial commit for new VMD device driver

[Keith Busch]

Hi Bjorn,

Thanks for the feedback. Much of the issues you mentioned look pretty
straight forward to resolve, and will fix of for the next revision.

I have some immediate follow up comments to two issues you brought up:

On Tue, 6 Oct 2015, Bjorn Helgaas wrote:
> > +static int vmd_find_free_domain(void)
> > +{
> > +	int domain = -1;
> > +	struct pci_bus *bus = NULL;
> > +
> > +	while ((bus = pci_find_next_bus(bus)) != NULL)
> > +		domain = max_t(int, domain, pci_domain_nr(bus));
> > +	if (domain < 0)
> > +		return -ENODEV;
> > +	return domain + 1;
> > +}
>
> The PCI core should own domain number allocation.  We have a little
> bit of generic domain support, e.g., pci_get_new_domain_nr().  On x86,
> I think that is compiled-in, but currently not used -- currently x86
> only uses _SEG from ACPI.  How would you handle collisions between a
> domain number assigned here (or via pci_get_new_domain_nr()) and a
> hot-added host bridge where _SEG uses the same domain?

Thank you for bringing this up as I hadn't thought much about it and may
have misunderstood the meaning of _SEG. AIUI, it is used to identify a
logical grouping. The OS does not need to use the same identifier for
the domain, so we there's no need to collide if we can assign the domain
of a a new _SEG to the next available domain_nr.

On pci_get_new_domain_nr(), can we make this use something like an
ida instead of an atomic? I think we'd like to put domains back into
the available pool if someone unbinds it. I imagine someone will test
unbinding/rebinding these devices in a loop for a while, and will file
a bug after the atomic increments to 64k. :)

> > +	resource_list_for_each_entry(entry, &resources) {
> > +		struct resource *source, *resource = entry->res;
> > +
> > +		if (!i) {
> > +			resource->start = 0;
> > +			resource->end = (resource_size(
> > +					&vmd->dev->resource[0]) >> 20) - 1;
> > +			resource->flags = IORESOURCE_BUS | IORESOURCE_PCI_FIXED;
>
> I thought BAR0 was CFGBAR.  I missed the connection to a bus number
> aperture.

Right, BAR0 is the CFGBAR and is the device's aperture to access its
domain's config space. Based on your comment, I assume that's not
a bus resource, though it seems to work with the posted code. :)
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