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

[Zihan Yang]

Kevin O'Connor  于2018年9月28日周五 上午6:30写道：
>
> On Thu, Sep 27, 2018 at 11:05:13PM +0800, Zihan Yang wrote:
> > Laszlo Ersek  于2018年9月26日周三 上午1:17写道：
> > > 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
> [...]
> > Things might change in the future if we can figure out a better solution, 
> > and I
> > hope we can have an easier and more elegant solution in OVMF. But now
> > we are just trying to give a possible solution as a poc.
>
> Thanks.  I wasn't aware this was a proof of concept.  (Nor have I been
> following the discussions on the qemu list.)  I don't think it makes
> sense to merge this into the main SeaBIOS repository.  The
> QEMU/firmware interface is already complex and I don't think we should
> complicate it further without a more concrete use case.  In
> particular, it seems unclear if 256 buses is enough or if 1024 buses
> is too little.

Yes, 1024 is indeed an ambiguous bound for now, but can you elaborate how
concrete should the use case be? For example, do we need to know what
they are going to do with so many network/storage devices? Because I
think normal users rarely need so many devices, but those who really want
devices might not always be willing to tell us their internal project status.

Thanks
Zihan

> -Kevin

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

[Zihan Yang]

Dr. David Alan Gilbert  于2018年9月28日周五 上午1:53写道：
>
> * Zihan Yang (whois.zihan.y...@gmail.com) wrote:
> > HI Laszlo
> > Laszlo Ersek  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 [0x8000, 0xb000) 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

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