> On 05-Sep-2018, at 5:50 PM, Andrew Jones <drjo...@redhat.com> 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
