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

[Glauber Costa]

On Wed, Apr 8, 2009 at 2:02 PM, Jan Kiszka <address@hidden> wrote:
> Glauber Costa wrote:
>> 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
>
> Unless I misread something, this is about option ROMs after VGA ROM.
>
>> 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.
>>
>
> My ideas is to merge both ROM regions (given we are actually talking
> about compatible stuff here): the 2k VGA ROM would first be expanded to
> 4k, then the next option ROM starting after the VGA would extend the
> latter. But that's plain theory so far.
>
>> I believe being explicit, and requiring page alignment is safer, and
>> does not hurt.
>
> In this case, we would waste 2k of (sometimes) precious low mem...
>

If we can merge the regions, then I agree with you. We'll probably have an
alignment requirement anyway, but it can well be handled in kvm layer.

However, as you ack yourself, it is likely to take some time. In the mean time,
I believe this fix is acceptable. Unless you're planning on having something
to fix it in the next few days.


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

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