KVM: selftests: Verify KVM disable interception (for userspace) on filter change

Re-read MSR_{FS,GS}_BASE after restoring the "allow everything" userspace
MSR filter to verify that KVM stops forwarding exits to userspace. This
can also be used in conjunction with manual verification (e.g. printk) to
ensure KVM is correctly updating the MSR bitmaps consumed by hardware.

Tested-by: Dapeng Mi <[e-mail]>
Tested-by: Manali Shukla <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

tools/testing/selftests/kvm/x86/userspace_msr_exit_test.c

···
 	data = test_rdmsr(MSR_GS_BASE);
 	GUEST_ASSERT(data == MSR_GS_BASE);
 
+	/* Access the MSRs again to ensure KVM has disabled interception.*/
+	data = test_rdmsr(MSR_FS_BASE);
+	GUEST_ASSERT(data != MSR_FS_BASE);
+	data = test_rdmsr(MSR_GS_BASE);
+	GUEST_ASSERT(data != MSR_GS_BASE);
+
 	GUEST_DONE();
 }
 
···
 		    "Expected ucall state to be UCALL_SYNC.");
 	vm_ioctl(vm, KVM_X86_SET_MSR_FILTER, &filter_gs);
 	run_guest_then_process_rdmsr(vcpu, MSR_GS_BASE);
+
+	vm_ioctl(vm, KVM_X86_SET_MSR_FILTER, &filter_allow);
 	run_guest_then_process_ucall_done(vcpu);
 }