Re: KVM Processor cache size

[Anthony Liguori <anthony@codemonkey.ws>]

On 08/02/2010 08:08 AM, Avi Kivity wrote:
>  On 08/02/2010 03:51 PM, Andre Przywara wrote:
>> Ricardo Martins wrote:
>>> Hi guys,
>>>
>>> I'm having a problem with kvm, my physical machine have 2 processor
>>> Xeon E5520, with 8 mb cache size, when i run "cat /proc/cpuinfo" the
>>> linux shows 16 processors equal.
>>>
>>> processor       : 15
>>> vendor_id       : GenuineIntel
>>> cpu family      : 6
>>> model           : 26
>>> model name      : Intel(R) Xeon(R) CPU           E5520  @ 2.27GHz
>>> stepping        : 5
>>> cpu MHz         : 1596.000
>>> cache size      : 8192 KB
>> > ....
>>> The problem is that when I run the same command on the virtual
>>> machine, Linux shows the processors with only 32 kb, I believe that
>>> anything is wrong.
>>>
>>> processor       : 0
>>> vendor_id       : GenuineIntel
>>> cpu family      : 6
>>> model           : 6
>>> model name      : QEMU Virtual CPU version 0.9.1
>>> stepping        : 3
>>> cpu MHz         : 2266.575
>>> cache size      : 32 KB
>> > ....
>>> On virtual Machine:
>>> linea:/# x86info
>>> x86info v1.21.  Dave Jones 2001-2007
>>>
>>>
>>> Found 1 CPU, but found 16d CPUs in MPTable.
>>> -------------------------------------------------------------------------- 
>>>
>>> Family: 6 Model: 6 Stepping: 3 Type: 0 Brand: 0
>>> CPU Model: Celeron / Mobile Pentium II Original OEM
>>> Feature flags:
>>>  fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36
>>> clflsh mmx fxsr sse sse2
>>> Extended feature flags:
>>>  sse3 [31]
>>>  [0] [2] [3] [4] [5] [6] [7] [8] [9] SYSCALL [13] [15] [16] xd [23] 
>>> [24] em64t
>>> Cache info
>>>  L1 Instruction cache: 32KB, 8-way associative. 64 byte line size.
>>>  L1 Data cache: 32KB, 8-way associative. 64 byte line size.
>>>  L2 unified cache: 2MB, sectored, 8-way associative. 64 byte line size.
>>> TLB info
>>
>> KVM (or better: the QEMU part) injects a bogus CPU model (compare 
>> family/model/name), which is the same on all host CPUs. This helps 
>> with migration, because the CPU does not change.
>> The cache size is also the same, as it is part of the "cpuid" command 
>> output.
>> You can use other CPU models (like kvm64) for better base models, but 
>> the cache size will likely not match your host processor's one.
>> For that purpose exists the "host" CPU model, which will (to some 
>> degree) simply push your host CPU model to the guest. For now this 
>> only affects the family/model/stepping/name values and the feature 
>> flags.
>> 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.  It's impossible to use 
correctly today in a production environment if you care about reliably 
generating the same guest visible interface.

Regards,

Anthony Liguori