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:
m...@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
