Re: [PATCH 8/9] KVM/VMX: Allow direct access to MSR_IA32_SPEC_CTRL

On Tue, Feb 6, 2018 at 9:29 AM, David Woodhouse dwmw@amazon.co.uk wrote:

@@ -8946,6 +9017,27 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu) #endif );

•   /*
  

•    * We do not use IBRS in the kernel. If this vCPU has used the
  

•    * SPEC_CTRL MSR it may have left it on; save the value and
  

•    * turn it off. This is much more efficient than blindly adding
  

•    * it to the atomic save/restore list. Especially as the former
  

•    * (Saving guest MSRs on vmexit) doesn't even exist in KVM.
  

•    *
  

•    * For non-nested case:
  

•    * If the L01 MSR bitmap does not intercept the MSR, then we need to
  

•    * save it.
  

•    *
  

•    * For nested case:
  

•    * If the L02 MSR bitmap does not intercept the MSR, then we need to
  

•    * save it.
  

•    */
  

•   if (!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL))
  

•           rdmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
  

•   if (vmx->spec_ctrl)
  

•           wrmsrl(MSR_IA32_SPEC_CTRL, 0);
  

Again, we haven't verified host support for this MSR. Perhaps this should be something like:

if (!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL) && !rdmsrl_safe(MSR_IA32_SPEC_CTRL, &vmx->spec_ctrl) && vmx->spec_ctrl) wrmsrl(MSR_IA32_SPEC_CTRL, 0);