Re: [Qemu-devel] different IDTs of the same VCPU

[Paolo Bonzini]

Il 13/03/2014 13:59, Alexander Binun ha scritto:
> Dear Friends,
>
>    Thanks for your assistance!
>
> We would like to ask you a question about the KVM internals.
>
> Our module includes a timer which (once in every second) fetches the IDT value of 
> every online VCPU in the system using the kvm_x86_ops->get_idt ; the code looks 
> like:
>
>   struct kvm_vcpu *curr_vcpu;
>   struct desc_ptr dt;
>
>   list_for_each_entry(kvm, vms_list, vm_list)
>   {
>     for (i = 0; i < kvm->online_vcpus.counter; i++)
>        {
>        curr_vcpu = kvm->vcpus[i];
>        kvm_x86_ops->get_idt(curr_vcpu, &dt);
>     }
>   }
>
> We have noticed that get_idt returns DIFFERENT values for the same
> VCPU (i.e. for the same value of i that refers to a given VCPU). We
> cannot understand this issue; could you explain ?
>
> It is very strange since nobody changes the IDT value (as , for example, 
> rootkits do).

At the very least, running nested virtualization would lead to different 
IDT values.

But more simply, on Intel you can hardly do anything with kvm_x86_ops or 
kvm_vcpu except on the same physical CPU that is in vcpu->cpu.  The 
state is not in memory, it is cached inside the physical CPU.

There is no easy solution to this without modifying KVM.  You can add a 
request bit to KVM's vcpu->requests field, kick the vcpu and do the 
check in vcpu_enter_guest.

Paolo