[Qemu-devel] Re: start qemu failed with --enable-kvm -vga std

[Jan Kiszka <jan.kiszka@siemens.com>]

Glauber Costa wrote:
> On Wed, Apr 8, 2009 at 10:12 AM, Anthony Liguori <anthony@codemonkey.ws> wrote:
>> Peng Huang wrote:
>>> Hi,
>>>
>>> The HEAD version qemu can not execute a VM with --enable-kvm -vga std or
>>> -vga vmware on kernel 2.6.29.1-46.fc11.x86_64. I found it is because of qemu
>>> call cpu_register_physical_memory with a wrong size. Below patch can fix it
>>> on my box. Please test it.
>> Glauber came up with a similar patch for kvm-userspace and is currently
>> attempting to root cause the issue.
> 
> I believe this is in fact the root cause. KVM slot management code
> probably require
> memory to be page aligned. By trying to register a region that is not
> page aligned,
> the ioctl may fail. I, however, did not see this happening in qemu
> upstream (only kvm-userspace),
> and have absolutely no idea about why. But the patch makes perfect sense to me.
> 

But this really sounds like a limitation that should better be fixed in
the kvm layer, not the device/machine code.

We only map ROM regions here. So rounding them up/down and sending
properly aligned requests to the kernel should finally have the same
result for all involved pages. Only if non-compatible regions overlap
due to such roundings, we should bail out - and start to consider
changing device code.

My next free time slot is reserved now for the anyway required
enhancement of kvm's slot management in qemu. kvm-userspace could catch
up afterwards (finally fixing its broken text console after reset...).

Jan

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