Re: [PATCH v2 6/6] KVM: nVMX: re-evaluate emulation_required on nested VM exit

[Sean Christopherson]

On Mon, Aug 30, 2021, Maxim Levitsky wrote:
> If L1 had invalid state on VM entry (can happen on SMM transactions
> when we enter from real mode, straight to nested guest),
> 
> then after we load 'host' state from VMCS12, the state has to become
> valid again, but since we load the segment registers with
> __vmx_set_segment we weren't always updating emulation_required.

Because I'm an idiot.

> Update emulation_required explicitly at end of load_vmcs12_host_state.

Can you also add

  Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>

which is how I ended up here, sort of.

Fixes: 816be9e9be8d ("KVM: nVMX: Don't evaluate "emulation required" on nested VM-Exit")

> Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> ---

The below is overkill.  The state after VM-Exit _can't_ be invalid, which was the
whole point of moving to __vmx_set_segment(), I just forgot the minor detail of
clearing emulation_required.  So just:

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index bc6327950657..55ac7211fb37 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -4314,6 +4314,12 @@ static void load_vmcs12_host_state(struct kvm_vcpu *vcpu,
        if (nested_vmx_load_msr(vcpu, vmcs12->vm_exit_msr_load_addr,
                                vmcs12->vm_exit_msr_load_count))
                nested_vmx_abort(vcpu, VMX_ABORT_LOAD_HOST_MSR_FAIL);
+
+       /*
+        * All relevant vmcs12 host state is checked prior to VM-Entry, thus
+        * L1 guest can never be invalid after VM-Exit.
+        */
+       to_vmx(vcpu)->emulation_required = false;
 }
 
 static inline u64 nested_vmx_get_vmcs01_guest_efer(struct vcpu_vmx *vmx)

>  arch/x86/kvm/vmx/nested.c | 2 ++
>  arch/x86/kvm/vmx/vmx.c    | 8 ++++----
>  arch/x86/kvm/vmx/vmx.h    | 1 +
>  3 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 1a05ae83dae5..f915e1ac589c 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -4319,6 +4319,8 @@ static void load_vmcs12_host_state(struct kvm_vcpu *vcpu,
>  	if (nested_vmx_load_msr(vcpu, vmcs12->vm_exit_msr_load_addr,
>  				vmcs12->vm_exit_msr_load_count))
>  		nested_vmx_abort(vcpu, VMX_ABORT_LOAD_HOST_MSR_FAIL);
> +
> +	to_vmx(vcpu)->emulation_required = vmx_emulation_required(vcpu);
>  }
>  
>  static inline u64 nested_vmx_get_vmcs01_guest_efer(struct vcpu_vmx *vmx)
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 02d061f5956a..7ff1e1daeb0a 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1323,7 +1323,7 @@ static void vmx_vcpu_put(struct kvm_vcpu *vcpu)
>  	vmx_prepare_switch_to_host(to_vmx(vcpu));
>  }
>  
> -static bool emulation_required(struct kvm_vcpu *vcpu)
> +bool vmx_emulation_required(struct kvm_vcpu *vcpu)
>  {
>  	return emulate_invalid_guest_state && !vmx_guest_state_valid(vcpu);
>  }
> @@ -1367,7 +1367,7 @@ void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
>  	vmcs_writel(GUEST_RFLAGS, rflags);
>  
>  	if ((old_rflags ^ vmx->rflags) & X86_EFLAGS_VM)
> -		vmx->emulation_required = emulation_required(vcpu);
> +		vmx->emulation_required = vmx_emulation_required(vcpu);
>  }
>  
>  u32 vmx_get_interrupt_shadow(struct kvm_vcpu *vcpu)
> @@ -3077,7 +3077,7 @@ void vmx_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
>  	}
>  
>  	/* depends on vcpu->arch.cr0 to be set to a new value */
> -	vmx->emulation_required = emulation_required(vcpu);
> +	vmx->emulation_required = vmx_emulation_required(vcpu);
>  }
>  
>  static int vmx_get_max_tdp_level(void)
> @@ -3330,7 +3330,7 @@ static void vmx_set_segment(struct kvm_vcpu *vcpu, struct kvm_segment *var, int
>  {
>  	__vmx_set_segment(vcpu, var, seg);
>  
> -	to_vmx(vcpu)->emulation_required = emulation_required(vcpu);
> +	to_vmx(vcpu)->emulation_required = vmx_emulation_required(vcpu);
>  }
>  
>  static void vmx_get_cs_db_l_bits(struct kvm_vcpu *vcpu, int *db, int *l)
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index 4858c5fd95f2..3a587c51a8d1 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -359,6 +359,7 @@ void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu);
>  void vmx_set_host_fs_gs(struct vmcs_host_state *host, u16 fs_sel, u16 gs_sel,
>  			unsigned long fs_base, unsigned long gs_base);
>  int vmx_get_cpl(struct kvm_vcpu *vcpu);
> +bool vmx_emulation_required(struct kvm_vcpu *vcpu);
>  unsigned long vmx_get_rflags(struct kvm_vcpu *vcpu);
>  void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags);
>  u32 vmx_get_interrupt_shadow(struct kvm_vcpu *vcpu);
> -- 
> 2.26.3
>