Re: [Qemu-devel] [Query] Live Migration between machines with different processor ids

[Jaggi, Manish]

> On 05-Sep-2018, at 6:11 PM, Jaggi, Manish <address@hidden> wrote:
> 
> 
> 
>> On 05-Sep-2018, at 5:50 PM, Andrew Jones <address@hidden> wrote:
>> 
>> External Email
>> 
>> On Wed, Sep 05, 2018 at 11:46:11AM +0000, Jaggi, Manish wrote:
>>> (a) Changes in KVM:
>>> 
>>> - Introducing a specific error code (KVM_EINVARIANT) in case of invariant 
>>> writes.
>>> This should not change anything to API SET_ONE_REG KVM API.
>>> Not sure which is the best place to put the define…
>>> I have added in include/uapi/linux/kvm_para.h.
>>> 
>>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>>> index 22fbbdbece3c..c8a4fbe8a8bb 100644
>>> --- a/arch/arm64/kvm/sys_regs.c
>>> +++ b/arch/arm64/kvm/sys_regs.c
>>> @@ -1111,7 +1111,7 @@ static int __set_id_reg(const struct sys_reg_desc 
>>> *rd, void __user *uaddr,
>>> 
>>>       /* This is what we mean by invariant: you can't change it. */
>>>       if (val != read_id_reg(rd, raz))
>>> -               return -EINVAL;
>>> +               return -KVM_EINVARIANT;
>>> 
>>>       return 0;
>>> }
>>> diff --git a/include/uapi/linux/kvm_para.h b/include/uapi/linux/kvm_para.h
>>> index 6c0ce49931e5..3a49a321d0df 100644
>>> --- a/include/uapi/linux/kvm_para.h
>>> +++ b/include/uapi/linux/kvm_para.h
>>> @@ -17,6 +17,7 @@
>>> #define KVM_E2BIG              E2BIG
>>> #define KVM_EPERM              EPERM
>>> #define KVM_EOPNOTSUPP         95
>>> +#define KVM_EINVARIANT          96
>>> 
>>> #define KVM_HC_VAPIC_POLL_IRQ          1
>>> #define KVM_HC_MMU_OP                  2
>>> 
>>> (b) Changes in Qemu code
>>> 
>>> 1. Handling of new error code, which would update
>>> guest state with hosts invariant reg values.
>>> 
>>> diff --git a/target/arm/kvm.c b/target/arm/kvm.c
>>> index 65f867d569..0cf14323a2 100644
>>> --- a/target/arm/kvm.c
>>> +++ b/target/arm/kvm.c
>>> @@ -452,7 +452,15 @@ bool write_list_to_kvmstate(ARMCPU *cpu, int level)
>>>            abort();
>>>        }
>>>        ret = kvm_vcpu_ioctl(cs, KVM_SET_ONE_REG, &r);
>>> -        if (ret) {
>>> +        if (ignore_invariant && (ret == -KVM_EINVARIANT)) {
>>> +            /* Update Guest invariant to match with migrated host regs*/
>>> +            ret = kvm_vcpu_ioctl(cs, KVM_GET_ONE_REG, &r);
>>> +            if (ret)
>>> +                ok = false;
>>> +            else
>>> +                cpu->cpreg_values[i] = r.addr;
>>> +        }
>>> +        else if (ret) {
>>>            /* We might fail for "unknown register" and also for
>>>             * "you tried to set a register which is constant with
>>>             * a different value from what it actually contains".
>>> 
>>> 2. ignore_invariant is the flag I was referring to which is what you also
>>> mentioned opt-in. This can be supplied as a command line parameter to qemu 
>>> on Machine B.
>> 
>> The same libvirt folk also suggest that a QMP command be provided that
>> allows the selection of this ignore-invariant mode (in addition to or
>> instead of the command line parameter.) Otherwise a guest that has
>> already started without the parameter will not be able to migrate to
>> a "close enough” host
> 
> ignore_invariant: This can be supplied as a command line parameter to qemu on 
> Machine B.
> Machine A VM has to be migrated to B.
> 
>> - even if it's decided later that it would be OK
>> to do so.
>> 
>>> 
>>> PS: I will add code to put warning logs as suggested by Dave.
>> 
>> Yeah, I like that idea too.
>> 
>> This approach looks good to me. Let's see what maintainers say when they
>> see the patch submission.

The patch was submiited to the list.
Please have a look 
https://lists.gnu.org/archive/html/qemu-devel/2018-09/msg03546.html
> 
> Cool. Thanks.
>> 
>> Thanks,
>> drew