[SeaBIOS] [RFC v3] pciinit: setup mcfg for pxb-pcie to support multiple pci domains

[Zihan Yang]

Dr. David Alan Gilbert <dgilbert at redhat.com> 于2018年9月28日周五 上午1:53写道：
>
> * Zihan Yang (whois.zihan.yang at gmail.com) wrote:
> > HI Laszlo
> > Laszlo Ersek <lersek at redhat.com> 于2018年9月26日周三 上午1:17写道：
> > >
> > > On 09/25/18 17:38, Kevin O'Connor wrote:
> > > > On Mon, Sep 17, 2018 at 11:02:59PM +0800, Zihan Yang wrote:
> > > >> To support multiple pci domains of pxb-pcie device in qemu, we need to setup
> > > >> mcfg range in seabios. We use [0x80000000, 0xb0000000) to hold new domain mcfg
> > > >> table for now, and we need to retrieve the desired mcfg size of each pxb-pcie
> > > >> from a hidden bar because they may not need the whole 256 busses, which also
> > > >> enables us to support more domains within a limited range (768MB)
> > > >
> > > > At a highlevel, this looks okay to me.  I'd like to see additional
> > > > reviews from others more familiar with the QEMU PCI code, though.
> > > >
> > > > Is the plan to do the same thing for OVMF?
> > >
> > > I remain entirely unconvinced that this feature is useful. (I've stated
> > > so before.)
> > >
> > > I believe the latest QEMU RFC posting (v5) is here:
> > >
> > > [Qemu-devel] [RFC v5 0/6] pci_expander_brdige: support separate pci
> > > domain for pxb-pcie
> > >
> > > http://mid.mail-archive.com/1537196258-12581-1-git-send-email-whois.zihan.yang@gmail.com
> > >
> > > First, I fail to see the use case where ~256 PCI bus numbers aren't
> > > enough. If I strain myself, perhaps I can imagine using ~200 PCIe root
> > > ports on Q35 (each of which requires a separate bus number), so that we
> > > can independently hot-plug 200 devices then. And that's supposedly not
> > > enough, because we want... 300? 400? A thousand? Doesn't sound realistic
> > > to me. (This is not meant to be a strawman argument, I really have no
> > > idea what the feature would be useful for.)
> >
> > It might not be very intuitive, but it indeed exists. The very
> > beginning discussion
> > about 4 months ago has mentioned a possible use case, and I paste it here
> >
> > - We have Ray from Intel trying to use 1000 virtio-net devices
>
> why that many?

I think I'll cc Marcel for the details but I remember that he is not aware of
Intel's purpose either. A guess would be that Intel has NFV, DPDK projects,
maybe they are playing some magic on the network? I wish I knew, but I
doubt whether Intel would let us know their internal projects.

> > - We may have a VM managing some backups (tapes), we may have a lot of these.
>
> I'm curious; what does tape backup have to do with the number of PCI
> slots/busses?

I'm not very clear about how tape works in qemu, but the problem is pcie
devices under q35. The pcie topology requires one device per bus, therefore
the 256 bus might not be enough if we have many pcie devices. Current
pxb-pcie still resides in domain 0, and is limited by the number 256. I think
he means these tape devices would consume all the available busses
we have in domain 0.

Thanks
Zihan