[PATCH 09/37] KVM: arm64: Move debug dirty flag calculation out of world switch

[Andrew Jones]

On Thu, Oct 12, 2017 at 12:41:13PM +0200, Christoffer Dall wrote:
> There is no need to figure out inside the world-switch if we should
> save/restore the debug registers or not, we can might as well do that in
> the higher level debug setup code, making it easier to optimize down the
> line.
> 
> Signed-off-by: Christoffer Dall <christoffer.dall at linaro.org>
> ---
>  arch/arm64/kvm/debug.c        | 9 +++++++++
>  arch/arm64/kvm/hyp/debug-sr.c | 6 ------
>  2 files changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index dbadfaf..62550de19 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -193,6 +193,15 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>  	if (trap_debug)
>  		vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;
>  
> +	/*
> +	 * If any of KDE, MDE or KVM_ARM64_DEBUG_DIRTY is set, perform
> +	 * a full save/restore cycle.

The commit message implies testing KVM_ARM64_DEBUG_DIRTY, but it only
tests KDE and MDE.

> +	 */
> +	if ((vcpu_sys_reg(vcpu, MDSCR_EL1) & DBG_MDSCR_KDE) ||
> +	    (vcpu_sys_reg(vcpu, MDSCR_EL1) & DBG_MDSCR_MDE))

nit: could also write as

 if (vcpu_sys_reg(vcpu, MDSCR_EL1) & (DBG_MDSCR_KDE | DBG_MDSCR_MDE))

> +		vcpu->arch.debug_flags |= KVM_ARM64_DEBUG_DIRTY;
> +

It looks like there's only one flag for debug_flags - this dirty flag,
which I guess is also used to trigger trapping. So maybe this could be a
second flag of a "lazy state" field, as I suggested earlier?

> +
>  	trace_kvm_arm_set_dreg32("MDCR_EL2", vcpu->arch.mdcr_el2);
>  	trace_kvm_arm_set_dreg32("MDSCR_EL1", vcpu_sys_reg(vcpu, MDSCR_EL1));
>  }
> diff --git a/arch/arm64/kvm/hyp/debug-sr.c b/arch/arm64/kvm/hyp/debug-sr.c
> index f5154ed..0fc0758 100644
> --- a/arch/arm64/kvm/hyp/debug-sr.c
> +++ b/arch/arm64/kvm/hyp/debug-sr.c
> @@ -172,12 +172,6 @@ void __hyp_text __debug_restore_state(struct kvm_vcpu *vcpu,
>  
>  void __hyp_text __debug_cond_save_host_state(struct kvm_vcpu *vcpu)
>  {
> -	/* If any of KDE, MDE or KVM_ARM64_DEBUG_DIRTY is set, perform
> -	 * a full save/restore cycle. */
> -	if ((vcpu->arch.ctxt.sys_regs[MDSCR_EL1] & DBG_MDSCR_KDE) ||
> -	    (vcpu->arch.ctxt.sys_regs[MDSCR_EL1] & DBG_MDSCR_MDE))
> -		vcpu->arch.debug_flags |= KVM_ARM64_DEBUG_DIRTY;
> -
>  	__debug_save_state(vcpu, &vcpu->arch.host_debug_state.regs,
>  			   kern_hyp_va(vcpu->arch.host_cpu_context));
>  	__debug_save_spe()(&vcpu->arch.host_debug_state.pmscr_el1);
> -- 
> 2.9.0
>

Also, while grepping, I noticed __debug_cond_restore_host_state() unsets
KVM_ARM64_DEBUG_DIRTY on that condition that it's set. A micro
optimization could be to do it unconditionally, removing the branch.

Thanks,
drew