cpu instruction CMPXCHG16B is disable

[airychkov]

cpu instruction CMPXCHG16B is disable need for special software

[vasily Levchenko]

Could you please attach the log?

[Ramshankar Venkataraman]

Which special software are you talking about?

[thebug]

We're having the same issue with our main application. I could attach a logfile but it will not show you anything as we check the cx16 flag of the cpu properties, and complain if it is not set. We use a linux kernel as a base and in /proc/cpuinfo this flag is not set, although the real cpu we're running on supports the instruction (the application runs fine with just the linux kernel without any virtualization).

I can imagine that more applications are having a problem with this. Notice that KVM and VMware do support te instruction, so we can run on these platforms. We need this instruction for performance reasons, simulating it by using a mutex is out of the question so this is a major issue for us.

[ShaXbee]

Quick answer: Set flag VBoxInternal/CPUM/CMPXCHG16B to true via

VBoxManage setextradata [vmname] VBoxInternal/CPUM/CMPXCHG16B 1

After searching for root cause I found out that problem is caused by bug in CPUM.cpp:

    /** @cfgm{/CPUM/CMPXCHG16B, boolean, false}
     * Expose CMPXCHG16B to the guest if supported by the host.
     */
    bool fCmpXchg16b;
    rc = CFGMR3QueryBoolDef(pCpumCfg, "CMPXCHG16B", &fCmpXchg16b, false); AssertRCReturn(rc, rc);

    [...]

    | (fCmpXchg16b ? X86_CPUID_FEATURE_ECX_CX16 : 0)

Comment suggests that flag will be true if host CPU supports instruction but default value is false.

[Frank Mehnert]

I will mark this as duplicate of #11899.