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

["Michael S. Tsirkin" <mst@redhat.com>]

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.

For example, if you want kvm to work with live migration you must either
use least common denominator cpu id, or use hardware from the same
vendor.

-- 
MST