[Qemu-devel] qemu 2.4 machine + kernel 4.2 + kvm : freeze & cpu 100% at start on some hardware (maybe KVM_CAP_X86_SMM related)

[Alexandre DERUMIER]

Hi,

I'm currently implementing qemu 2.4 for proxmox hypervisors,
and a lot of users have reported qemu freeze with cpu at 100% when starting.
Connecting with vnc display : "qemu guest has not initialized the display yet"

Similar bug report here : 
https://lacyc3.eu/qemu-guest-has-not-initialized-the-display


This does not occur on all hardware, 
for example it freeze on dell powerege r710  (xeon E5540),  but not on dell 
r630 (CPU E5-2687W v3 @ 3.10GHz)
or very old dell poweredge 2950 (xeon 5110  @ 1.60GHz).

This is only with qemu 2.4 + kernel 4.2 (kernel 4.1 works fine) + kvm




not working command line
-------------------------
/usr/bin/kvm chardev 
socket,id=qmp,path=/var/run/qemu-server/100.qmp,server,nowait -mon 
chardev=qmp,mode=control -vnc unix:/var/run/qemu-server/100.vnc,x509,password 
-pidfile /var/run/qemu-server/100.pid -name test -cpu kvm64 -m 4096 -machine 
pc-i440fx-2.4


working command line
---------------------
qemu 2.4 + kvm + compat 2.3 profil:

/usr/bin/kvm chardev 
socket,id=qmp,path=/var/run/qemu-server/100.qmp,server,nowait -mon 
chardev=qmp,mode=control -vnc unix:/var/run/qemu-server/100.vnc,x509,password 
-pidfile /var/run/qemu-server/100.pid -name test -cpu kvm64 -m 4096 -machine 
pc-i440fx-2.3

qemu 2.4 without kvm:

/usr/bin/kvm chardev 
socket,id=qmp,path=/var/run/qemu-server/100.qmp,server,nowait -mon 
chardev=qmp,mode=control -vnc unix:/var/run/qemu-server/100.vnc,x509,password 
-pidfile /var/run/qemu-server/100.pid -name test -cpu kvm64 -m 4096 -machine 
accel=tcg,type=pc-i440fx-2.4



So it's working with qemu 2.4 + machine 2.3 compat profil.



Looking at the code:

static void pc_compat_2_3(MachineState *machine)
{
    PCMachineState *pcms = PC_MACHINE(machine);
    savevm_skip_section_footers();
    if (kvm_enabled()) {
        pcms->smm = ON_OFF_AUTO_OFF;
    }
    global_state_set_optional();
    savevm_skip_configuration();
}


If I comment 
//pcms->smm = ON_OFF_AUTO_OFF;

I have the same freeze too.



So,it's seem to come from somewhere in


bool pc_machine_is_smm_enabled(PCMachineState *pcms)
{
    bool smm_available = false;

    if (pcms->smm == ON_OFF_AUTO_OFF) {
        return false;
    }

    if (tcg_enabled() || qtest_enabled()) {
        smm_available = true;
    } else if (kvm_enabled()) {
        smm_available = kvm_has_smm();    >> maybe here ?
    }

    if (smm_available) {
        return true;
    }

    if (pcms->smm == ON_OFF_AUTO_ON) {
        error_report("System Management Mode not supported by this 
hypervisor.");
        exit(1);
    }
    return false;
}


bool kvm_has_smm(void)
{
    return kvm_check_extension(kvm_state, KVM_CAP_X86_SMM);
}



I'm not sure if it's a qemu bug or kernel/kvm bug.

Help is welcome.


Regards,

Alexandre Derumier