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Date: Mon, 14 Dec 2009 13:44:22 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] cpuid problem in upstream qemu with kvm
References: <20091214193541.GA6150@redhat.com>
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: qemu-devel@nongnu.org, gleb@redhat.com, avi@redhat.com

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