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

[Jaggi, Manish]

> 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.

Cool. Thanks.
> 
> Thanks,
> drew