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Re: [Qemu-devel] [Xen-devel] [BUG 1747]Guest could't find bootable devic

From: Stefano Stabellini
Subject: Re: [Qemu-devel] [Xen-devel] [BUG 1747]Guest could't find bootable device with memory more than 3600M
Date: Tue, 11 Jun 2013 18:26:43 +0100
User-agent: Alpine 2.02 (DEB 1266 2009-07-14)

On Mon, 10 Jun 2013, George Dunlap wrote:
> On Sat, Jun 8, 2013 at 8:27 AM, Hao, Xudong <address@hidden> wrote:
> >>
> >> What happens on real hardware when the BIOS boots and finds out that the
> >> PCI hole is too small to contain all the MMIO regions of the PCI devices?
> >>
> >
> > I do not know what does BIOS do details, but I think it's in 4G above if 
> > pci hole is too small. I saw native OS program the pci hole to a very high 
> > addr 0x3c0c00000000 if the pci device has large bar size.
> >
> > For Xen hvmloader, it already policy that is: if PCI hole is too small, 
> > another MMIO region will be created starting from the high_mem_pgend.
> Yes, and that, as I understand it, is the problem: that this change of
> the MMIO regions is not communicated to qemu.
> But it seems like this same problem would occur on real hardware --
> i.e., you add a new video card with 2GiB of video ram, and now you
> need a 2.5 GiB MMIO hole. It seems like the BIOS would be the logical
> place to sort it out.  But I'm not familiar enough with SeaBIOS / qemu
> to really know for sure.

I went through the code that maps the PCI MMIO regions in hvmloader
(tools/firmware/hvmloader/pci.c:pci_setup) and it looks like it already
maps the PCI region to high memory if the PCI bar is 64-bit and the MMIO
region is larger than 512MB.

Maybe we could just relax this condition and map the device memory to
high memory no matter the size of the MMIO region if the PCI bar is
If the Nvidia Quadro that originally Gonglei was trying to assign to the
guest is 64-bit capable, then it would fix the issue in the simplest
possible way.

Are there actually any PCI devices that people might want to assign to
guests that don't have 64-bit bars and don't fit in

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