Re: [PATCH v2 4/7] KVM: x86: allow kvm_x86_ops.set_efer to return a value

[Jim Mattson]

On Thu, Aug 20, 2020 at 6:34 AM Maxim Levitsky <mlevitsk@xxxxxxxxxx> wrote:
>
> This will be used later to return an error when setting this msr fails.
>
> For VMX, it already has an error condition when EFER is
> not in the shared MSR list, so return an error in this case.
>
> Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> ---

> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1471,7 +1471,8 @@ static int set_efer(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>         efer &= ~EFER_LMA;
>         efer |= vcpu->arch.efer & EFER_LMA;
>
> -       kvm_x86_ops.set_efer(vcpu, efer);
> +       if (kvm_x86_ops.set_efer(vcpu, efer))
> +               return 1;

This seems like a userspace ABI change to me. Previously, it looks
like userspace could always use KVM_SET_MSRS to set MSR_EFER to 0 or
EFER_SCE, and it would always succeed. Now, it looks like it will fail
on CPUs that don't support EFER in hardware. (Perhaps it should fail,
but it didn't before, AFAICT.)