Re: [PATCH 1/2] kvm: vmx: Use basic exit reason to check if it's the specific VM EXIT

[Xiaoyao Li]

On 2/25/2020 12:17 AM, Sean Christopherson wrote:
> On Mon, Feb 24, 2020 at 02:04:46PM +0100, Vitaly Kuznetsov wrote:
> > Xiaoyao Li <xiaoyao.li@xxxxxxxxx> writes:
> >
> > > On 2/24/2020 6:16 PM, Vitaly Kuznetsov wrote:
> > > > Xiaoyao Li <xiaoyao.li@xxxxxxxxx> writes:
> >
> > ...
> >
> > > > >    		rip = kvm_rip_read(vcpu);
> > > > >    		rip += vmcs_read32(VM_EXIT_INSTRUCTION_LEN);
> > > > >    		kvm_rip_write(vcpu, rip);
> > > > > @@ -5797,6 +5797,7 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu,
> > > > >    {
> > > > >    	struct vcpu_vmx *vmx = to_vmx(vcpu);
> > > > >    	u32 exit_reason = vmx->exit_reason;
> > > > > +	u16 basic_exit_reason = basic(exit_reason);
> > > >
> > > > I don't think renaming local variable is needed, let's just do
> > > >
> > > > 'u16 exit_reason = basic_exit_reason(vmx->exit_reason)' and keep the
> > > > rest of the code as-is.
> > >
> > > No, we can't do this.
> > >
> > > It's not just renaming local variable, the full 32-bit exit reason is
> > > used elsewhere in this function that needs the upper 16-bit.
> > >
> > > Here variable basic_exit_reason is added for the cases where only basic
> > > exit reason number is needed.
> >
> > Can we do the other way around, i.e. introduce 'extended_exit_reason'
> > and use it where all 32 bits are needed? I'm fine with the change, just
> > trying to minimize the (unneeded) code churn.
>
> 100% agree.  Even better than adding a second field to vcpu_vmx would be
> to make it a union, though we'd probably want to call it something like
> full_exit_reason in that case.  That should give us compile-time checks on
> exit_reason, e.g. if we try to query one of the upper bits using a u16, e.g.

I have thought about union, but it seems

union {
	u16 exit_reason;
	u32 full_exit_reason;
}

is not a good name. Since there are many codes in vmx.c and nested.c 
assume that exit_reason stands for 32-bit EXIT REASON vmcs field as well 
as evmcs->vm_exit_reason and vmcs12->vm_exit_reason. Do we really want 
to also rename them to full_exit_reason?

Maybe we name it

union {
	u16 basic_exit_reason;
	u32 exit_reason;
}

as what SDM defines?

> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5818,7 +5818,7 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu,
>          if (is_guest_mode(vcpu) && nested_vmx_exit_reflected(vcpu, exit_reason))
>                  return nested_vmx_reflect_vmexit(vcpu, exit_reason);
>
> -       if (exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY) {
> +       if (vmx->full_exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY) {
>                  dump_vmcs();
>                  vcpu->run->exit_reason = KVM_EXIT_FAIL_ENTRY;
>                  vcpu->run->fail_entry.hardware_entry_failure_reason
> @@ -6620,11 +6620,12 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>          vmx->nested.nested_run_pending = 0;
>          vmx->idt_vectoring_info = 0;
>
> -       vmx->exit_reason = vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON);
> -       if ((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY)
> +       vmx->full_exit_reason = vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON);
> +       if (vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY)
>                  kvm_machine_check();
>
> -       if (vmx->fail || (vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
> +       if (vmx->fail ||
> +           (vmx->full_exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
>                  return;
>
>          vmx->loaded_vmcs->launched = 1;
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index 7f42cf3dcd70..60c09640ea59 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -260,7 +260,10 @@ struct vcpu_vmx {
>          int vpid;
>          bool emulation_required;
>
> -       u32 exit_reason;
> +       union {
> +               u16 exit_reason;
> +               u32 full_exit_reason;
> +       }
>
>          /* Posted interrupt descriptor */
>          struct pi_desc pi_desc;
>
>
>
>
>
> > --
> > Vitaly