[libvirt] [Qemu-devel] [PATCH/QEMU] s390x/kvm: use cpu_model_available for guarded storage on compat machines

Halil Pasic pasic at linux.vnet.ibm.com
Fri Oct 27 12:31:00 UTC 2017

On 10/25/2017 08:13 PM, Jason J. Herne wrote:
> On 10/20/2017 10:54 AM, Christian Borntraeger wrote:
>> Starting a guest with
>>     <os>
>>      <type arch='s390x' machine='s390-ccw-virtio-2.9'>hvm</type>
>>    </os>
>>    <cpu mode='host-model'/>
>> on an IBM z14 results in
>> "qemu-system-s390x: Some features requested in the CPU model are not
>> available in the configuration: gs"
>> This is because guarded storage is fenced for compat machines that did not have
>> guarded storage support, but libvirt expands the cpu model according to the
>> latest available machine.
>> While this prevents future migration abort (by not starting the guest at all),
>> not being able to start a "host-model" guest is very much unexpected.  As it
>> turns out, even if we would modify libvirt to not expand the cpu model to
>> contain "gs" for compat machines, it cannot guarantee that a migration will
>> succeed. For example if the kernel changes its features (or the user has
>> nested=1 on one host but not on the other) the migration will fail
>> nevertheless.  So instead of fencing "gs" for machines <= 2.9 lets allow it for
>> all machine types that support the CPU model. This will make "host-model"
>> runnable all the time, while relying on the CPU model to reject invalid
>> migration attempts.
> ...
>> -    if (gs_allowed()) {
>> +    if (cpu_model_allowed()) {
>>           if (kvm_vm_enable_cap(s, KVM_CAP_S390_GS, 0) == 0) {
>>               cap_gs = 1;


Hi Jason,

I don't have access to a z14 at the moment, and since you do, I would
like to try out something.

I will first describe my concern, and then the test scenario.

The last line above, cap_gs = 1, has the side effect of returning
true ever after.

int kvm_s390_get_gs(void)                                                       
    return cap_gs;                                                              

Now considering
static bool gscb_needed(void *opaque)
    return kvm_s390_get_gs();

const VMStateDescription vmstate_gscb = {
    .name = "cpu/gscb",
    .version_id = 1,
    .minimum_version_id = 1,
    .needed = gscb_needed,
    .fields = (VMStateField[]) {
        VMSTATE_UINT64_ARRAY(env.gscb, S390CPU, 4),

const VMStateDescription vmstate_s390_cpu = {
    .name = "cpu",
    .post_load = cpu_post_load,
    .pre_save = cpu_pre_save,
    .version_id = 4,
    .minimum_version_id = 3,
    .fields      = (VMStateField[]) {
        VMSTATE_UINT64_ARRAY(env.regs, S390CPU, 16),
        VMSTATE_UINT64(env.psw.mask, S390CPU),
        VMSTATE_UINT64(env.psw.addr, S390CPU),
        VMSTATE_UINT64(env.psa, S390CPU),
        VMSTATE_UINT32(env.todpr, S390CPU),
        VMSTATE_UINT64(env.pfault_token, S390CPU),
        VMSTATE_UINT64(env.pfault_compare, S390CPU),
        VMSTATE_UINT64(env.pfault_select, S390CPU),
        VMSTATE_UINT64(env.cputm, S390CPU),
        VMSTATE_UINT64(env.ckc, S390CPU),
        VMSTATE_UINT64(env.gbea, S390CPU),
        VMSTATE_UINT64(env.pp, S390CPU),
        VMSTATE_UINT32_ARRAY(env.aregs, S390CPU, 16),
        VMSTATE_UINT64_ARRAY(env.cregs, S390CPU, 16),
        VMSTATE_UINT8(env.cpu_state, S390CPU),
        VMSTATE_UINT8(env.sigp_order, S390CPU),
        VMSTATE_UINT32_V(irqstate_saved_size, S390CPU, 4),
        VMSTATE_VBUFFER_UINT32(irqstate, S390CPU, 4, NULL,
    .subsections = (const VMStateDescription*[]) {

I would expect the vmstate_gscb subsection being sent, even if gs is disabled
via cpu-model if kernel and possibly machine has gs support (and qemu
has cpu-models).

So the test scenario I want you to play trough is the following. Take
the latest-greatest qemu with this patch applied. Make sure gs works
(is provided to the guest) with a 2.9 machine version, and a fully
specified cpu-model. Now disable gs explicitly.

Try to migrate this to another machine having a 2.9 binary. I expect
the migration failing because, the subsection is going to be sent by
the latest-greatest binary, but is unknown to the 2.9 binary.

Notice this is despite the fact that gs is explicitly disabled.

Now that I think about it, maybe the 2.9 binary is going to reject
the explicit gs flag altogether, because it's unknown.

Isn't this a problem? I'm afraid like this the only migration-safe
variant is -base, but that would essentially make adding features
incrementally impossible. 

But I hypothesize trying to migrate with z13 or even z13-base
would also trigger the unknown subsection problem.

Unfortunately I can't test this because my kernel never
makes kvm_vm_enable_cap(s, KVM_CAP_S390_GS, 0) return 0, because
I lack HW support in the host.


> Ok, honestly, I dislike this idea because it means we are effectively lying now. We will happily accept a +gs cpu model with a 2.9 compat machine when the point of compat machines is to mimick the older version of Qemu which does not support GS.  Yes, model checking will prevent the worst side effects, namely, exploding migrations.
> I'd far prefer a solution that makes host-model dependent on the machine type. But I understand some of the backlash on that idea. Still, it seems like the cleanest approach even if it will be more work.
> With all of that said, I know we want this fixed and your patch seems to fix the problem. So if you need an R-b...
> Reviewed-by: Jason J. Herne <jjherne at linux.vnet.ibm.com>
> Can we have a new tag? :-D
> Reviewed-by-with-reservations: Jason J. Herne <jjherne at linux.vnet.ibm.com>

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