Re: [PATCH v2 17/24] KVM: nVMX: Add a helper to identify low-priority #DB traps

[Maxim Levitsky]

On Fri, 2022-07-15 at 20:42 +0000, Sean Christopherson wrote:
> Add a helper to identify "low"-priority #DB traps, i.e. trap-like #DBs
> that aren't TSS T flag #DBs, and tweak the related code to operate on any
> queued exception.  A future commit will separate exceptions that are
> intercepted by L1, i.e. cause nested VM-Exit, from those that do NOT
> trigger nested VM-Exit.  I.e. there will be multiple exception structs
> and multiple invocations of the helpers.
> 
> No functional change intended.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/vmx/nested.c | 23 +++++++++++++++++------
>  1 file changed, 17 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index a0a4eddce445..c3fc8b484785 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3859,14 +3859,24 @@ static void nested_vmx_inject_exception_vmexit(struct kvm_vcpu *vcpu,
>   * from the emulator (because such #DBs are fault-like and thus don't trigger
>   * actions that fire on instruction retire).
>   */
> -static inline unsigned long vmx_get_pending_dbg_trap(struct kvm_vcpu *vcpu)
> +static unsigned long vmx_get_pending_dbg_trap(struct kvm_queued_exception *ex)
>  {
> -       if (!vcpu->arch.exception.pending ||
> -           vcpu->arch.exception.vector != DB_VECTOR)
> +       if (!ex->pending || ex->vector != DB_VECTOR)
>                 return 0;
>  
>         /* General Detect #DBs are always fault-like. */
> -       return vcpu->arch.exception.payload & ~DR6_BD;
> +       return ex->payload & ~DR6_BD;
> +}
> +
> +/*
> + * Returns true if there's a pending #DB exception that is lower priority than
> + * a pending Monitor Trap Flag VM-Exit.  TSS T-flag #DBs are not emulated by
> + * KVM, but could theoretically be injected by userspace.  Note, this code is
> + * imperfect, see above.
> + */
> +static bool vmx_is_low_priority_db_trap(struct kvm_queued_exception *ex)
> +{
> +       return vmx_get_pending_dbg_trap(ex) & ~DR6_BT;
>  }
>  
>  /*
> @@ -3878,8 +3888,9 @@ static inline unsigned long vmx_get_pending_dbg_trap(struct kvm_vcpu *vcpu)
>   */
>  static void nested_vmx_update_pending_dbg(struct kvm_vcpu *vcpu)
>  {
> -       unsigned long pending_dbg = vmx_get_pending_dbg_trap(vcpu);
> +       unsigned long pending_dbg;
>  
> +       pending_dbg = vmx_get_pending_dbg_trap(&vcpu->arch.exception);
>         if (pending_dbg)
>                 vmcs_writel(GUEST_PENDING_DBG_EXCEPTIONS, pending_dbg);
>  }
> @@ -3949,7 +3960,7 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
>          * prioritize SMI over MTF and trap-like #DBs.
>          */
>         if (vcpu->arch.exception.pending &&
> -           !(vmx_get_pending_dbg_trap(vcpu) & ~DR6_BT)) {
> +           !vmx_is_low_priority_db_trap(&vcpu->arch.exception)) {
>                 if (block_nested_exceptions)
>                         return -EBUSY;
>                 if (!nested_vmx_check_exception(vcpu, &exit_qual))


Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky