Re: [PATCH 3/4] kvm-unit-test: nVMX: __enter_guest() should not set "launched" state when VM-entry fails

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Fri, Sep 13, 2019 at 01:37:55PM -0700, Krish Sadhukhan wrote:
> 
> On 9/4/19 8:42 AM, Sean Christopherson wrote:
> >On Thu, Aug 29, 2019 at 04:56:34PM -0400, Krish Sadhukhan wrote:
> >>Bit# 31 in VM-exit reason is set by hardware in both cases of early VM-entry
> >>failures and VM-entry failures due to invalid guest state.
> >This is incorrect, VMCS.EXIT_REASON is not written on a VM-Fail.  If the
> >tests are passing, you're probably consuming a stale EXIT_REASON.
> 
> In vmx_vcpu_run(),
> 
>         if (vmx->fail || (vmx->exit_reason &
> VMX_EXIT_REASONS_FAILED_VMENTRY))
>                 return;
> 
>         vmx->loaded_vmcs->launched = 1;
> 
> we return without setting "launched" whenever bit# 31 is set in Exit Reason.
> If VM-entry fails due to invalid guest state or due to errors in VM-entry
> MSR-loading area, bit#31 is set.  As a result, L2 is not in "launched" state
> when we return to L1.  Tests that want to VMRESUME L2 after fixing the bad
> guest state or the bad MSR-loading area, fail with VM-Instruction Error 5,
> 
>         "Early vmresume failure: error number is 5. See Intel 30.4."

Yes, a VMCS isn't marked launched if it generates a VM-Exit due to a
failed consistency check.  But as that code shows, a failed consistency
check results in said VM-Exit *or* a VM-Fail.  Cosnsitency checks that
fail early, i.e. before loading guest state, generate VM-Fail, any check
that fails after the CPU has started loading guest state manifests as a
VM-Exit.  VMCS.EXIT_REASON isn't touched in the VM-Fail case.

E.g. in CHECKS ON VMX CONTROLS AND HOST-STATE AREA, the SDM states:

  VM entry fails if any of these checks fail.  When such failures occur,
  control is passed to the next instruction, RFLAGS.ZF is set to 1 to
  indicate the failure, and the VM-instruction error field is loaded with
  an error number that indicates whether the failure was due to the
  controls or the host-state area (see Chapter 30).