Difference between vcpu_load and kvm_sched_in ?

[Yacine <y_hebbal@esi.dz>]

Hi, I'm a student working on virtual machine introspection.

I'm trying to implement an application on top of KVM in which I need to trap
writes to CR3 (host with 8 cores and guest with one vcpu).

When I do this when handling a VM EXIT using:
vmcs_set_bits(CPU_BASED_VM_EXEC_CONTROL, CPU_BASED_CR3_LOAD_EXITING), it
works correctly and I can see the traps in my log file.

Now when I do the same thing after receiving a command from Qemu (command is
handled in kvm_vm_ioctl by calling a function I added to kvm_x86_ops
vmx_x86_ops) I get a vmwrite error. I found out that the problem is because
the logical processor on the host that is handling the ioctl command is not
the same that is running the VM and holding its state; so I must do the
vmwrite on the one executing the VM

To change the logical cpu executing the VM, I tried this:

vcpu_load; start cr3 trapping; vcpu_put

it worked correctly (in my logs I see that vcpu.cpu become equal to "cpu =
raw_smp_processor_id();") but the VM blocks for a lot of time due to mutex
in vcpu_load (up to serveral seconds and sometimes minutes !)

I replaced vcpu_load with kvm_sched_in, now everything works perfectly and
the VM doesn't block at all (logs here: http://pastebin.com/h5XNNMcb).

So, what I want to know is: what is the difference between vcpu_load and
kvm_sched_in ? both of this functions call kvm_arch_vcpu_loadbut the latter
one does it without doing a mutex

Is there a problem in using kvm_sched_in instead of vcpu_load for my use case ?