linux-kernel - Re: [RFC PATCH 1/3] kvm: x86: Rename KVM_DEBUGREG_RELOAD to KVM_DEBUGREG_NEED

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Message-ID: <20200423190941.GN17824@linux.intel.com>
Date:   Thu, 23 Apr 2020 12:09:42 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        Nadav Amit <namit@...technion.ac.il>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/3] kvm: x86: Rename KVM_DEBUGREG_RELOAD to
 KVM_DEBUGREG_NEED_RELOAD

On Thu, Apr 16, 2020 at 06:15:07PM +0800, Xiaoyao Li wrote:
> To make it more clear that the flag means DRn (except DR7) need to be
> reloaded before vm entry.
> 
> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
> ---
>  arch/x86/include/asm/kvm_host.h | 2 +-
>  arch/x86/kvm/x86.c              | 6 +++---
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index c7da23aed79a..f465c76e6e5a 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -511,7 +511,7 @@ struct kvm_pmu_ops;
>  enum {
>  	KVM_DEBUGREG_BP_ENABLED = 1,
>  	KVM_DEBUGREG_WONT_EXIT = 2,
> -	KVM_DEBUGREG_RELOAD = 4,
> +	KVM_DEBUGREG_NEED_RELOAD = 4,

My vote would be for KVM_DEBUGREG_DIRTY  Any bit that is set switch_db_regs
triggers a reload, whereas I would expect a RELOAD flag to be set _every_
time a load is needed and thus be the only bit that's checked

>  };
>  
>  struct kvm_mtrr_range {
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index de77bc9bd0d7..cce926658d10 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1067,7 +1067,7 @@ static void kvm_update_dr0123(struct kvm_vcpu *vcpu)
>  	if (!(vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP)) {
>  		for (i = 0; i < KVM_NR_DB_REGS; i++)
>  			vcpu->arch.eff_db[i] = vcpu->arch.db[i];
> -		vcpu->arch.switch_db_regs |= KVM_DEBUGREG_RELOAD;
> +		vcpu->arch.switch_db_regs |= KVM_DEBUGREG_NEED_RELOAD;
>  	}
>  }
>  
> @@ -8407,7 +8407,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  		set_debugreg(vcpu->arch.eff_db[2], 2);
>  		set_debugreg(vcpu->arch.eff_db[3], 3);
>  		set_debugreg(vcpu->arch.dr6, 6);
> -		vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
> +		vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_NEED_RELOAD;
>  	}
>  
>  	kvm_x86_ops.run(vcpu);
> @@ -8424,7 +8424,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  		kvm_update_dr0123(vcpu);
>  		kvm_update_dr6(vcpu);
>  		kvm_update_dr7(vcpu);
> -		vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
> +		vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_NEED_RELOAD;

This is the path that I think would really benefit from DIRTY, it took me
several reads to catch that kvm_update_dr0123() will set RELOAD.

>  	}
>  
>  	/*
> -- 
> 2.20.1
>