Re: [Qemu-devel] [Bug 1297651] [NEW] KVM create a win7 guest with Qemu, it boots up fail

[Laszlo Ersek]

On 03/26/14 16:23, Eduardo Habkost wrote:
> On Wed, Mar 26, 2014 at 05:09:29PM +0200, Michael S. Tsirkin wrote:
>> On Wed, Mar 26, 2014 at 12:06:38PM -0300, Eduardo Habkost wrote:
>>> On Wed, Mar 26, 2014 at 04:54:31PM +0200, Michael S. Tsirkin wrote:
>>>> On Wed, Mar 26, 2014 at 02:48:29PM +0100, Igor Mammedov wrote:
>>> [...]
>>>>> more clean would be to abort if CPON index (i.e. APIC ID)
>>>>> is more than 255. That would affect small number of weird
>>>>> topologies but sould be fine for most usecases.
>>>>
>>>> Any idea how to trigger this behaviour?
>>>
>>> Set sockets and/or cores to a 2^n+1 value. It will use n+1 bits for the
>>> socket ID and core ID, making APIC ID larger than CPU index. You can get
>>> a very large APIC ID if you use, for example:
>>>
>>>   -smp 1,cores=17,threads=17,maxcpus=200
>>>
>>> There's code to limit the maximum APIC ID and abort if it is too large,
>>> already (so the above command-line will make QEMU abort).
>>
>> Okay so how do you make this package size exactly 256?
> 
> I didn't look for a proof yet, but it looks like having apic_id_limit
> exactly 256 while max_cpus <= 255 is impossible.
> 
> Laszlo, did you find a case where this was possible?

No, it's impossible, like you say; see my other email with the small
program doing an exhaustive search. Any topology that simultaneously
results in:
- a maximal VCPU count <= 255, and
- an APIC ID width <= 8
will prevent the exact APIC ID value 255.

Thanks
Laszlo