Re: KVM Processor cache size

[Avi Kivity <avi@redhat.com>]

  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.

-- 
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