2017-05-29 15:24+0200, Gioh Kim:
> Current code sets unusable as 1 if present is 1 and type is 0.
> In Long mode, type value in segment descriptor is ignored.
> So I think type should be ignored when setting the segment registers,
> if type means the descriptor type in the segment descriptor.
>
> Is the type field of struct kvm_segment the descriptor type?
Yes.
> If so, why type is checked when setting segment registers?
No idea. 19bca6ab75d8 ("KVM: SVM: Fix cross vendor migration issue with
unusable bit") also moved the assigment up to initialize it before use
and I think that is enough.
> If the type field is not the descriptor type,
> is it ok to set unusable when present is 1?
Looks like a bug. type = 0 can be a usable read-only data segment.
> I'm copying a code as following to show what code I'm asking.
Please send it as a patch,
thanks.
> ----------------------------- 8< ---------------------------------
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 5f48f62..0133f6f 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1803,7 +1803,7 @@ static void svm_get_segment(struct kvm_vcpu *vcpu,
> * AMD's VMCB does not have an explicit unusable field, so emulate it
> * for cross vendor migration purposes by "not present"
> */
> - var->unusable = !var->present || (var->type == 0);
> + var->unusable = !var->present;
>
> switch (seg) {
> case VCPU_SREG_TR:
> --
> 2.5.0
>
