Re: [PATCH] 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.
> 
> Signed-off-by: Avi Kivity <avi@redhat.com>
> ---
>  arch/x86/kvm/vmx.c |    6 ++++--
>  1 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 05cd554..70b0c54 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -3629,7 +3629,8 @@ 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 +3732,8 @@ 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)

That reduces the emulation quality as vcpu->arch.switch_db_regs is only
set if some breakpoint is active while dr6 has its use also when that is
not the case).

Jan

-- 
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