Re: [Qemu-devel] [RFC v2 1/3] pci_expander_bridge: add type TYPE_PXB_PCIE_HOST

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

On Wed, Jun 13, 2018 at 04:23:40PM +0800, Zihan Yang wrote:
> Michael S. Tsirkin <mst@redhat.com> 于2018年6月12日周二 下午9:43写道：
> >
> > On Tue, Jun 12, 2018 at 05:13:22PM +0800, Zihan Yang wrote:
> > > The inner host bridge created by pxb-pcie is TYPE_PXB_PCI_HOST by default,
> > > add a new type TYPE_PXB_PCIE_HOST to better utilize the ECAM of PCIe
> > >
> > > Signed-off-by: Zihan Yang <whois.zihan.yang@gmail.com>
> >
> > I have a concern that there are lots of new properties
> > added here, I'm not sure how are upper layers supposed to
> > manage them all.
> >
> > E.g. bus_nr supplied in several places, domain_nr for which
> > it's not clear how it is supposed to be allocated, etc.
> 
> Indeed they seem to double the properties, but the pxb host is
> an internal structure of pxb-pcie device, created in pxb-pcie's
> realization procedure, and acpi-build queries host bridges instead
> of pxb-pcie devices. This means that users can not directly specify
> the property of pxb host bridge, but must 'inherit' from pxb-pcie
> devices. I had thought about changing the acpi-build process,
> but that would require more modifications.
> As for the properties, bus_nr means the start bus number
> of this host bridge. It is used when pxb-pcie is in pci domain 0
> with q35 host to avoid bus number confliction. When it is placed
> in a separate pci domain, it is not used and should be 0.
> 
> max_bus means how many buses the user desires, EACH bus in
> PCIe requires 1MB configuration space, thus specifying it could
> reduce the reserved memory in MMCFG as suggested by Marcel.
> Typically, the user can specify
> 
> -device pxb-pcie,id=br1,bus="pcie.0",sep_domain=on,domain_nr=1,max_bus=130
> 
> this will place the buses under this pxb host bridge in pci domain
> 1, and reserve (130 + 1) = 131 buses for it. The start bus number
> is always 0 currently for simplicity.
> 
> > Can the management interface be simplified?
> > Ideally we wouldn't have to teach libvirt new tricks,
> > just generalize pxb support slightly.
> 
> We can delete 'sep_domain' property, I just find 'domain_nr'
> already indicates domain number. But domain_nr and
> max_bus seems unremovable, although they look 'redundant'
> because they appear twice.
> 
> I'm not familiar with libvirt, but from the perspective of user,
> only 2 properties are added(domain_nr and max_bus, if we
> delete sep_domain), though the internal structure actually has
> changed.

If you want a property for an internal purpose,
you can have a property starting with "x-" this way
we don't commit to maintaining it.

-- 
MST