Re: KVM Processor cache size

[Andre Przywara <andre.przywara@amd.com>]

Anthony Liguori wrote:
> On 08/02/2010 08:08 AM, Avi Kivity wrote:
>>  On 08/02/2010 03:51 PM, Andre Przywara wrote:
>>> I sent a patch to include the cache size when using -cpu host, but 
>>> this has been n'acked because the benefit is not clear.
>> Anthony, why was this NACKed?  First, there are programs which query 
>> the cache size.  That's why it's exposed!  Second, -cpu host is for 
>> exposing as many host cpu features as we can, not just those we have 
>> an immediate use for.  It's like 'cp -a' dropping attributes the 
>> author didn't care about.
> 
> That's exactly what the code does today BTW.  The kernel module filters 
> cpuid flags, qemu filters additional flags, and we don't pass everything 
> through anyway.
> 
> -cpu host is a mess and needs some love.
The mentioned patch addresses this to some degree, where it creates a 
list of CPUID leafs which can be safely passed through. There are some 
which we should not (and sometimes must not) propagate (think of host 
topology and power management), but these should not block the useful 
ones. CPUID transports a lot of different information, so we should also 
distinguish here.

> It's impossible to use 
> correctly today in a production environment if you care about reliably 
> generating the same guest visible interface.
Do you mean by this that the guest sees a different CPU model on each 
host it is started? Actually this is a default scenario for native 
machines and CPUID was introduced for applications to adapt to this.
So if you leave migration aside, -cpu host is actually the natural way.

Regards,
Andre.

-- 
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 488-3567-12