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

[Glauber Costa]

On Wed, Apr 8, 2009 at 1:34 PM, Jan Kiszka <address@hidden> wrote:
> Glauber Costa wrote:
>> On Wed, Apr 8, 2009 at 10:12 AM, Anthony Liguori <address@hidden> 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.

I've considered this, but then the alignment constraints become implicit,
rather than explicit. Consider the option rom registration code itself.

We should start loading option roms right after vga. If kvm layer
aligns the region
implicitly, then where will option roms start? At better, we would have to tell
qemu back that such an alignment were made, to start looking for option
roms in the right place.

I believe being explicit, and requiring page alignment is safer, and
does not hurt.

-- 
Glauber  Costa.
"Free as in Freedom"
http://glommer.net

"The less confident you are, the more serious you have to act."