Re: [Qemu-devel] [SeaBIOS] [RFC v2 0/3] Support multiple pci domains in pci_device

[Zihan Yang <whois.zihan.yang@gmail.com>]

Gerd Hoffmann <kraxel@redhat.com> 于2018年8月27日周一 上午7:04写道：
>
>   Hi,
>
> > >   However, QEMU only binds port 0xcf8 and 0xcfc to
> > > bus pcie.0. To avoid bus confliction, we should use other port pairs for
> > > busses under new domains.
> >
> > I would skip support for IO based configuration and use only MMCONFIG
> > for extra root buses.
> >
> > The question remains: how do we assign MMCONFIG space for
> > each PCI domain.
>
> Allocation-wise it would be easiest to place them above 4G.  Right after
> memory, or after etc/reserved-memory-end (if that fw_cfg file is
> present), where the 64bit pci bars would have been placed.  Move the pci
> bars up in address space to make room.
>
> Only problem is that seabios wouldn't be able to access mmconfig then.
>
> Placing them below 4G would work at least for a few pci domains.  q35
> mmconfig bar is placed at 0xb0000000 -> 0xbfffffff, basically for
> historical reasons.  Old qemu versions had 2.75G low memory on q35 (up
> to 0xafffffff), and I think old machine types still have that for live
> migration compatibility reasons.  Modern qemu uses 2G only, to make
> gigabyte alignment work.
>
> 32bit pci bars are placed above 0xc0000000.  The address space from 2G
> to 2.75G (0x8000000 -> 0xafffffff) is unused on new machine types.
> Enough room for three additional mmconfig bars (full size), so four
> pci domains total if you add the q35 one.

Maybe we can support 4 domains first before we come up
with a better solution. But I'm not sure if four domains are
enough for those who want too many devices?

> cheers,
>   Gerd
>