linux-kernel - Re: [PATCH 4/7] KVM: vmx: Allow the guest to run with dirty debug registers

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Message-ID: <20140309182639.GA10479@potion.redhat.com>
Date:	Sun, 9 Mar 2014 19:26:39 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	alex.williamson@...hat.com, mtosatti@...hat.com, gleb@...nel.org,
	jan.kiszka@...mens.com
Subject: Re: [PATCH 4/7] KVM: vmx: Allow the guest to run with dirty debug
 registers

2014-03-07 12:42+0100, Paolo Bonzini:
> When not running in guest-debug mode (i.e. the guest controls the debug
> registers, having to take an exit for each DR access is a waste of time.
> If the guest gets into a state where each context switch causes DR to be
> saved and restored, this can take away as much as 40% of the execution
> time from the guest.
> 
> If the guest is running with vcpu->arch.db == vcpu->arch.eff_db, we
> can let it write freely to the debug registers and reload them on the
> next exit.  We still need to exit on the first access, so that the
> KVM_DEBUGREG_WONT_EXIT flag is set in switch_db_regs; after that, further
> accesses to the debug registers will not cause a vmexit.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/vmx.c | 37 ++++++++++++++++++++++++++++++++++++-
>  1 file changed, 36 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 06e4ec877a1c..e377522408b1 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -2843,7 +2843,7 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf)
>  		      vmx_capability.ept, vmx_capability.vpid);
>  	}
>  
> -	min = 0;
> +	min = VM_EXIT_SAVE_DEBUG_CONTROLS;
>  #ifdef CONFIG_X86_64
>  	min |= VM_EXIT_HOST_ADDR_SPACE_SIZE;
>  #endif
> @@ -5113,6 +5113,22 @@ static int handle_dr(struct kvm_vcpu *vcpu)
>  		}
>  	}
>  
> +	if (vcpu->guest_debug == 0) {
> +		u32 cpu_based_vm_exec_control;
> +
> +		cpu_based_vm_exec_control = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL);
> +		cpu_based_vm_exec_control &= ~CPU_BASED_MOV_DR_EXITING;
> +		vmcs_write32(CPU_BASED_VM_EXEC_CONTROL, cpu_based_vm_exec_control);

vmcs_clear_bits() covers exactly this use-case.
(Barring the explicit bit-width.)

> +
> +		/*
> +		 * No more DR vmexits; force a reload of the debug registers
> +		 * and reenter on this instruction.  The next vmexit will
> +		 * retrieve the full state of the debug registers.
> +		 */
> +		vcpu->arch.switch_db_regs |= KVM_DEBUGREG_WONT_EXIT;
> +		return 1;
> +	}
> +

We could make the code slighly uglier and move the functional part of
this block before the previous one, so it would do both things in one
exit.  (Exception handler will likely access DR too.)

>  	exit_qualification = vmcs_readl(EXIT_QUALIFICATION);
>  	dr = exit_qualification & DEBUG_REG_ACCESS_NUM;
>  	reg = DEBUG_REG_ACCESS_REG(exit_qualification);
> @@ -5139,6 +5155,24 @@ static void vmx_set_dr6(struct kvm_vcpu *vcpu, unsigned long val)
>  {
>  }
>  
> +static void vmx_sync_dirty_debug_regs(struct kvm_vcpu *vcpu)
> +{
> +	u32 cpu_based_vm_exec_control;
> +
> +	get_debugreg(vcpu->arch.db[0], 0);
> +	get_debugreg(vcpu->arch.db[1], 1);
> +	get_debugreg(vcpu->arch.db[2], 2);
> +	get_debugreg(vcpu->arch.db[3], 3);
> +	get_debugreg(vcpu->arch.dr6, 6);
> +	vcpu->arch.dr7 = vmcs_readl(GUEST_DR7);
> +
> +	vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_WONT_EXIT;
> +
> +	cpu_based_vm_exec_control = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL);
> +	cpu_based_vm_exec_control |= CPU_BASED_MOV_DR_EXITING;
> +	vmcs_write32(CPU_BASED_VM_EXEC_CONTROL, cpu_based_vm_exec_control);

(I'll be calling this sneaky later on.)

> +}
> +
>  static void vmx_set_dr7(struct kvm_vcpu *vcpu, unsigned long val)
>  {
>  	vmcs_writel(GUEST_DR7, val);
> @@ -8590,6 +8624,7 @@ static struct kvm_x86_ops vmx_x86_ops = {
>  	.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,
>  	.get_rflags = vmx_get_rflags,
>  	.set_rflags = vmx_set_rflags,
> -- 
> 1.8.3.1
> 
> 
> --
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