linux-kernel - Re: [PATCH 03/10] KVM: x86: Make kvm_x86_ops' {g,s}et

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Message-ID: <37a30f26-2561-aada-9bdc-7bc9cddf0d7f@redhat.com>
Date:   Mon, 4 May 2020 15:19:26 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/10] KVM: x86: Make kvm_x86_ops' {g,s}et_dr6() hooks
 optional

On 02/05/20 06:32, Sean Christopherson wrote:
> Make get_dr6() and set_dr6() optional and drop the VMX implementations,
> which are for all intents and purposes nops.  This avoids a retpoline on
> VMX when reading/writing DR6, at minimal cost (~1 uop) to SVM.

Can't get_dr6 be killed off completely here, since vcpu->arch.dr6 is the
only value that is ever passed to set_dr6?  OTOH no complaint about
adding the if for vmx_set_dr6, since that will also be covered nicely by
DEFINE_STATIC_COND_CALL.

Paolo

> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 11 -----------
>  arch/x86/kvm/x86.c     |  6 ++++--
>  2 files changed, 4 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index de18cd386bb1..e157bdc218ea 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5008,15 +5008,6 @@ static int handle_dr(struct kvm_vcpu *vcpu)
>  	return kvm_skip_emulated_instruction(vcpu);
>  }
>  
> -static u64 vmx_get_dr6(struct kvm_vcpu *vcpu)
> -{
> -	return vcpu->arch.dr6;
> -}
> -
> -static void vmx_set_dr6(struct kvm_vcpu *vcpu, unsigned long val)
> -{
> -}
> -
>  static void vmx_sync_dirty_debug_regs(struct kvm_vcpu *vcpu)
>  {
>  	get_debugreg(vcpu->arch.db[0], 0);
> @@ -7799,8 +7790,6 @@ static struct kvm_x86_ops vmx_x86_ops __initdata = {
>  	.set_idt = vmx_set_idt,
>  	.get_gdt = vmx_get_gdt,
>  	.set_gdt = vmx_set_gdt,
> -	.get_dr6 = vmx_get_dr6,
> -	.set_dr6 = vmx_set_dr6,
>  	.set_dr7 = vmx_set_dr7,
>  	.sync_dirty_debug_regs = vmx_sync_dirty_debug_regs,
>  	.cache_reg = vmx_cache_reg,
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8ec356ac1e6e..eccbfcb6a4e5 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1069,7 +1069,8 @@ static void kvm_update_dr0123(struct kvm_vcpu *vcpu)
>  
>  static void kvm_update_dr6(struct kvm_vcpu *vcpu)
>  {
> -	if (!(vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP))
> +	if (!(vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP) &&
> +	    kvm_x86_ops.set_dr6)
>  		kvm_x86_ops.set_dr6(vcpu, vcpu->arch.dr6);
>  }
>  
> @@ -1148,7 +1149,8 @@ int kvm_get_dr(struct kvm_vcpu *vcpu, int dr, unsigned long *val)
>  	case 4:
>  		/* fall through */
>  	case 6:
> -		if (vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP)
> +		if ((vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP) ||
> +		    !kvm_x86_ops.get_dr6)
>  			*val = vcpu->arch.dr6;
>  		else
>  			*val = kvm_x86_ops.get_dr6(vcpu);
>