Re: [PATCH v1 1/1] KVM: x86: Merge pending debug causes when vectoring #DB

[Sean Christopherson <seanjc@google.com>]

On Wed, May 13, 2026, Sean Christopherson wrote:
> On Wed, Jan 07, 2026, Aidan Khoury wrote:
> So while I don't exactly love the idea, I think this?  Compile tested only at
> this point, I'll try to properly test it tomorrow.

Confirmed the below works, once I remembered how to configure debug breakpoints.
I'll plan on sending a v2 on your behalf, along with a KVM-Unit-Test testcase.

> ---
>  arch/x86/include/asm/kvm-x86-ops.h | 1 +
>  arch/x86/include/asm/kvm_host.h    | 1 +
>  arch/x86/kvm/vmx/main.c            | 9 +++++++++
>  arch/x86/kvm/vmx/vmx.c             | 6 ++++++
>  arch/x86/kvm/vmx/x86_ops.h         | 1 +
>  arch/x86/kvm/x86.c                 | 8 ++++++++
>  6 files changed, 26 insertions(+)
> 
> diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> index b0269325646c..3e04a7ecbb47 100644
> --- a/arch/x86/include/asm/kvm-x86-ops.h
> +++ b/arch/x86/include/asm/kvm-x86-ops.h
> @@ -51,6 +51,7 @@ KVM_X86_OP(get_gdt)
>  KVM_X86_OP(set_gdt)
>  KVM_X86_OP(sync_dirty_debug_regs)
>  KVM_X86_OP(set_dr7)
> +KVM_X86_OP_OPTIONAL_RET0(get_pending_dbg_exceptions)
>  KVM_X86_OP(cache_reg)
>  KVM_X86_OP(get_rflags)
>  KVM_X86_OP(set_rflags)
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 271bdd109a98..eaac6ba59591 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1834,6 +1834,7 @@ struct kvm_x86_ops {
>  	void (*set_gdt)(struct kvm_vcpu *vcpu, struct desc_ptr *dt);
>  	void (*sync_dirty_debug_regs)(struct kvm_vcpu *vcpu);
>  	void (*set_dr7)(struct kvm_vcpu *vcpu, unsigned long value);
> +	unsigned long (*get_pending_dbg_exceptions)(struct kvm_vcpu *vcpu);
>  	void (*cache_reg)(struct kvm_vcpu *vcpu, enum kvm_reg reg);
>  	unsigned long (*get_rflags)(struct kvm_vcpu *vcpu);
>  	void (*set_rflags)(struct kvm_vcpu *vcpu, unsigned long rflags);
> diff --git a/arch/x86/kvm/vmx/main.c b/arch/x86/kvm/vmx/main.c
> index 83d9921277ea..a0e67471bff1 100644
> --- a/arch/x86/kvm/vmx/main.c
> +++ b/arch/x86/kvm/vmx/main.c
> @@ -470,6 +470,14 @@ static void vt_set_dr7(struct kvm_vcpu *vcpu, unsigned long val)
>  	vmx_set_dr7(vcpu, val);
>  }
>  
> +static unsigned long vt_get_pending_dbg_exceptions(struct kvm_vcpu *vcpu)
> +{
> +	if (WARN_ON_ONCE(is_td_vcpu(vcpu)))
> +		return 0;
> +
> +	return vmx_get_pending_dbg_exceptions(vcpu);
> +}
> +
>  static void vt_sync_dirty_debug_regs(struct kvm_vcpu *vcpu)
>  {
>  	/*
> @@ -928,6 +936,7 @@ struct kvm_x86_ops vt_x86_ops __initdata = {
>  	.get_gdt = vt_op(get_gdt),
>  	.set_gdt = vt_op(set_gdt),
>  	.set_dr7 = vt_op(set_dr7),
> +	.get_pending_dbg_exceptions = vt_op(get_pending_dbg_exceptions),
>  	.sync_dirty_debug_regs = vt_op(sync_dirty_debug_regs),
>  	.cache_reg = vt_op(cache_reg),
>  	.get_rflags = vt_op(get_rflags),
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 3a47a21dd499..7f84644a5c49 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5822,6 +5822,12 @@ void vmx_set_dr7(struct kvm_vcpu *vcpu, unsigned long val)
>  	vmcs_writel(GUEST_DR7, val);
>  }
>  
> +unsigned long vmx_get_pending_dbg_exceptions(struct kvm_vcpu *vcpu)
> +{
> +	return vmcs_readl(GUEST_PENDING_DBG_EXCEPTIONS) &
> +               (DR6_RTM | DR6_BS | DR_TRAP_BITS);
> +}
> +
>  static int handle_tpr_below_threshold(struct kvm_vcpu *vcpu)
>  {
>  	kvm_apic_update_ppr(vcpu);
> diff --git a/arch/x86/kvm/vmx/x86_ops.h b/arch/x86/kvm/vmx/x86_ops.h
> index 409858074246..5cfbd4e66f08 100644
> --- a/arch/x86/kvm/vmx/x86_ops.h
> +++ b/arch/x86/kvm/vmx/x86_ops.h
> @@ -75,6 +75,7 @@ void vmx_set_idt(struct kvm_vcpu *vcpu, struct desc_ptr *dt);
>  void vmx_get_gdt(struct kvm_vcpu *vcpu, struct desc_ptr *dt);
>  void vmx_set_gdt(struct kvm_vcpu *vcpu, struct desc_ptr *dt);
>  void vmx_set_dr7(struct kvm_vcpu *vcpu, unsigned long val);
> +unsigned long vmx_get_pending_dbg_exceptions(struct kvm_vcpu *vcpu);
>  void vmx_sync_dirty_debug_regs(struct kvm_vcpu *vcpu);
>  void vmx_cache_reg(struct kvm_vcpu *vcpu, enum kvm_reg reg);
>  unsigned long vmx_get_rflags(struct kvm_vcpu *vcpu);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 72ba1fc1bed2..8b32ab11f888 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -868,6 +868,14 @@ static void kvm_multiple_exception(struct kvm_vcpu *vcpu, unsigned int nr,
>  		vcpu->arch.exception.vector = nr;
>  		vcpu->arch.exception.error_code = error_code;
>  		vcpu->arch.exception.has_payload = has_payload;
> +		/*
> +		 * VMX records deferred debug causes (B0-B3, enabled breakpoint,
> +		 * BS, RTM) in the vmcs.PENDING_DBG_EXCEPTIONS field.  Merge any
> +		 * pending causes into the exception payload so the guest may
> +		 * see all accumulated reasons in DR6 when the #DB is vectored.
> +		 */
> +		if (nr == DB_VECTOR && has_payload)
> +			payload |= kvm_x86_call(get_pending_dbg_exceptions)(vcpu);
>  		vcpu->arch.exception.payload = payload;
>  		return;
>  	}
> 
> base-commit: b72a08c022f2dae4bca6f4edde5fe8012a02aefa
> --