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Re: [Qemu-devel] [RFC PATCH v2 05/21][SeaBIOS] pciinit: Fix pcimem_start

From: Gerd Hoffmann
Subject: Re: [Qemu-devel] [RFC PATCH v2 05/21][SeaBIOS] pciinit: Fix pcimem_start value
Date: Thu, 12 Jul 2012 09:22:14 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.5) Gecko/20120601 Thunderbird/10.0.5

On 07/11/12 18:45, Vasilis Liaskovitis wrote:
> Hi,
> On Wed, Jul 11, 2012 at 01:56:19PM +0200, Gerd Hoffmann wrote:
>> On 07/11/12 12:31, Vasilis Liaskovitis wrote:
>>> In order to hotplug memory between RamSize and BUILD_PCIMEM_START, the pci
>>> window needs to start at BUILD_PCIMEM_START (0xe0000000).
>>> Otherwise, the guest cannot online new dimms at those ranges due to pci_root
>>> window conflicts. (workaround for linux guest is booting with pci=nocrs)
>>>  static void pci_bios_map_devices(struct pci_bus *busses)
>>>  {
>>> -    pcimem_start = RamSize;
>>> +    pcimem_start = BUILD_PCIMEM_START;
>> It isn't that simple.  For the 32bit pci window it will work, but will
>> leaves address space unused instead of assigning it to the 32bit pci
>> window.  For the 64bit pci window it will not work.
>> You have to walk the dimms and figure what the highest used address is,
>> for both below-4g and above-4g.  Then fill two variable with it and make
>> the pci init code use that instead of RamSize and RamSizeOver4G.
> I see. I already have these values values computed in qemu-kvm, so I can pass
> them in a paravirt struct, or infer them from the dimm/srat paravirt info 
> that I
> already pass to seabios. 

I'd suggest to infer from the dimm info, to limit the amout of
information which needs to be passed from qemu to seabios.

> If i understand correctly, we would like the pcimem windows to use the maximum
> possible address space (constrained by the exact dimms/ranges which are 
> defined)
> instead of leaving unused space.

Yes, for the 32bit pci window.

The 64bit pci window is mapped above all memory, and it must likewise
consider defined+unfilled dimms so the start address doesn't collide
with memory hot-plugged above 4G later on.


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