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Re: [Qemu-devel] [PATCH] virtio-pci: make pci bar layout more flexible.

From: Gerd Hoffmann
Subject: Re: [Qemu-devel] [PATCH] virtio-pci: make pci bar layout more flexible.
Date: Wed, 04 Mar 2015 11:55:50 +0100


> > VirtIOPCIProxy subclasses which need additional pci bars, such as
> > virtio-vga, just need to make sure they register the additinal bars
> > before initializing virtio-pci, so the superclass can see the registered
> > bars and shuffle around the virtio bars accordingly.
> I think I prefer we just DTRT and keep same layouts for everyone by
> default: isn't there a layout that is good for everybody?

I want bar #2 for the vga framebuffer for virtio-vga.  Which conflicts
with bar #2 being used for the modern bar in todays code.

We can move the modern bar to #4 for everybody, then we'll have:

  #0 -- legacy i/o
  #1 -- msix
  #2 -- unused (by virtio)
  #3 -- unused (by virtio)
  #4 -- modern mem
  #5 -- modern mem too (because it's 64bit).

That'll leave bars #2 + #3 free, for additional bars (1x 64bit or 2x
32bit) such as vga framebuffer if needed.

> I know this means we'll leave BAR0 unused for modern devices but that
> does not seem too bad.

Yep, no technical reason against it, although it IMHO looks nicer.


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