[SeaBIOS] [Qemu-devel] [PATCH v2] map 64-bit PCI BARs at location provided by emulator

Gerd Hoffmann kraxel at redhat.com
Mon Oct 14 15:04:45 CEST 2013


> > 
> > To me it makes more sense to just go the direct route and say "please
> > put the 64bit bars at this location" rather than indirect "we might want
> > hotplug $thatmuch memory" and then expect the bios to leave that much
> > room.
> Only if the newfeature address is not under bios control.  I know that
> bios is simplistic so all it cares about ATM is pci window, but can't
> shake the impression that we are better off telling the guest what's
> going on rather than what it should do.
> In particular the issue that was discussed (what to do
> if pci start is set by host to below ram end)
> will simply go away if we pass in an incremental
> value: there will be no invalid configurations.

The "what is going on" might need updates in both qemu and seabios if
something new goes on.  For example qemu getting support non-contignous
memory.  The "leave that much address space free above memory" suddenly
is ambiguous as there are two (or more) memory blocks above 4g.  "please
place 64bit pci bars there" continues to work just fine.

> > > It's not unthinkable.
> > > Multiple ECAM regions (for multi-root systems) can make holes in the address space.
> > 
> > Sounds pretty theoretic ...
> What? Multiple PCI roots?

That we'll need pass multiple pci windows because of that.  I expect
we'll need seabios support for the new multi-root hardware anyway, and
seabios will probably be aware of the hardware constrains then ...

Also as far I know nobody is working on such a chipset.

I simply wouldn't worry about that today.  Designing a interface when
you don't know the exact needs for the it has a high chance to go wrong.

> > > Also, we just ignore everything above the ioapic, but that's
> > > not a must, we could maybe use address space above ioapic.
> > 
> > Any reason why we should that?
> > 
> 32 bit address space is contrained, using it is preferable for
> 32 bit guests ...

This wouldn't help in the cases I've seen in practice.  We have no
problems at all to fit in mmio pci bars, even lots of them, they are
small enough.  We can run out of address space if we have pci devices
with larger chunks of memory on them, such as qxl or ivshmem.  And these
memory bars are too big to fit into any of the small holes above the


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