Re: [PATCH v2] KVM: VMX: Conditionally reload debug register 6

[Jan Kiszka <jan.kiszka@siemens.com>]

Avi Kivity wrote:
> Only reload debug register 6 if we're running with the guest's
> debug registers.  Saves around 150 cycles from the guest lightweight
> exit path.
> 
> dr6 contains a couple of bits that are updated on #DB, so intercept
> that unconditionally and update those bits then.
> 
> Signed-off-by: Avi Kivity <avi@redhat.com>
> ---
> v2: trap #DB so we maintain the TF related bits of DR7.
> 
>  arch/x86/kvm/vmx.c |   14 +++++++++-----
>  1 files changed, 9 insertions(+), 5 deletions(-)
> 
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 05cd554..d4be978 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -540,10 +540,12 @@ static void update_exception_bitmap(struct kvm_vcpu *vcpu)
>  	eb = (1u << PF_VECTOR) | (1u << UD_VECTOR) | (1u << MC_VECTOR);
>  	if (!vcpu->fpu_active)
>  		eb |= 1u << NM_VECTOR;
> +	/*
> +	 * Unconditionally intercept #DB so we can maintain dr6 without
> +	 * reading it every exit.
> +	 */
> +	eb |= 1u << DB_VECTOR;

If this is safe...

>  	if (vcpu->guest_debug & KVM_GUESTDBG_ENABLE) {
> -		if (vcpu->guest_debug &
> -		    (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP))
> -			eb |= 1u << DB_VECTOR;
>  		if (vcpu->guest_debug & KVM_GUESTDBG_USE_SW_BP)
>  			eb |= 1u << BP_VECTOR;
>  	}
> @@ -3629,7 +3631,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  	 */
>  	vmcs_writel(HOST_CR0, read_cr0());
>  
> -	set_debugreg(vcpu->arch.dr6, 6);
> +	if (vcpu->arch.switch_db_regs)
> +		set_debugreg(vcpu->arch.dr6, 6);
>  
>  	asm(
>  		/* Store host registers */
> @@ -3731,7 +3734,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  				  | (1 << VCPU_EXREG_PDPTR));
>  	vcpu->arch.regs_dirty = 0;
>  
> -	get_debugreg(vcpu->arch.dr6, 6);
> +	if (vcpu->arch.switch_db_regs)
> +		get_debugreg(vcpu->arch.dr6, 6);
>  
>  	vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD);
>  	if (vmx->rmode.irq.pending)

...I wonder why we cannot drop this save/restore? The guest is not able
to access dr6 without causing a trap, thus will never see dr6 as stored
in the hardware.

Jan

-- 
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux