Re: [Qemu-devel] cpuid problem in upstream qemu with kvm

[Anthony Liguori <anthony@codemonkey.ws>]

Michael S. Tsirkin wrote:
> On Mon, Dec 14, 2009 at 01:44:22PM -0600, Anthony Liguori wrote:
>   
>> Michael S. Tsirkin wrote:
>>     
>>> Hi!
>>> I have a linux guest where init crashes during boot with invalid opcode error
>>> with kernel 2.6.31, but works fine with kernel 2.6.32.
>>> It also works fine even with 2.6.31 with qemu-kvm.
>>>
>>> With Gleb's help, we figured out that the opcode in question is syscall, which
>>> is unsupported by hardware on kvm with intel hosts, and is emulated in software
>>> with intel hosts in 2.6.32 and up. AMD hosts support this natively.
>>>
>>> The root cause: by default qemu with kvm enabled reports the following CPU ID:
>>>                mst@robin:~$ less /proc/cpuinfo
>>>                processor       : 0
>>>                vendor_id       : AuthenticAMD
>>>                cpu family      : 6
>>>                model           : 2
>>>                model name      : QEMU Virtual CPU version 0.11.50
>>>                stepping        : 3
>>>                cpu MHz         : 2493.679
>>>                cache size      : 512 KB
>>>                fdiv_bug        : no
>>>                hlt_bug         : no
>>>                f00f_bug        : no
>>>                coma_bug        : no
>>>                fpu             : yes
>>>                fpu_exception   : yes
>>>                cpuid level     : 4
>>>                wp              : yes
>>>                flags           : fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat
>>>                pse36 clflush mmx fxsr sse sse2 syscall lm up pni
>>>                bogomips        : 4987.35
>>>                clflush size    : 64
>>>                power management:
>>>
>>> Problem is, guest sees syscall+AMD vendor and concludes that it is ok to use
>>> syscall opcode, which results in a crash (or at least, extra VMExits).
>>> I understand from Avi that qemu-kvm solves this problem by reporting
>>> the actual CPU vendor from host, to guest.
>>>   
>>>       
>> The bit that's ugly about that is that we do not save/restore  
>> cpuid.vendor_id so that would break live migration.
>>
>> Regards,
>>
>> Anthony Liguori
>>     
>
> This is a feature.  It would be very wrong to save/restore cpuid IMO.
> It must be part of machine config.
>   

You can already do -cpu qemu64,vendor_id=AuthenticAMD or 
,vendor_id=GenuineIntel.

If you don't specify anything explicitly, we use AuthenticAMD as a 
default.  Detecting the host by default is problematic because then the 
same invocation on a different host would change the visible vendor_id.

I thought KVM emulates the syscall instruction?  I swear I've seen 
patches for that.

Regards,

Anthony Liguori