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 - 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.
Thanks,
drew
