Re: [PATCH] KVM: Use thread debug register storage instead of kvm specific data

[Avi Kivity <avi@redhat.com>]

On 09/01/2009 01:42 PM, Jan Kiszka wrote:
>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 891234b..9e3acbd 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -3627,14 +3627,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>
>>   	kvm_guest_enter();
>>
>> -	get_debugreg(vcpu->arch.host_dr6, 6);
>> -	get_debugreg(vcpu->arch.host_dr7, 7);
>>   	if (unlikely(vcpu->arch.switch_db_regs)) {
>> -		get_debugreg(vcpu->arch.host_db[0], 0);
>> -		get_debugreg(vcpu->arch.host_db[1], 1);
>> -		get_debugreg(vcpu->arch.host_db[2], 2);
>> -		get_debugreg(vcpu->arch.host_db[3], 3);
>> -
>>      
> I'm fine with this for the common case, but we should switch to the safe
> pattern on test_thread_flag(TIF_DEBUG), ie. if someone debugs qemu using
> hardware break/watchpoints.
>    

IIUC, thread.debugregs should be synced with the hardware registers.  
The kernel itself only writes to the debug registers, so we're safe 
doing the same.

>    
>>   		set_debugreg(0, 7);
>>   		set_debugreg(vcpu->arch.eff_db[0], 0);
>>   		set_debugreg(vcpu->arch.eff_db[1], 1);
>> @@ -3645,15 +3638,14 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>   	trace_kvm_entry(vcpu->vcpu_id);
>>   	kvm_x86_ops->run(vcpu);
>>
>> -	if (unlikely(vcpu->arch.switch_db_regs)) {
>> -		set_debugreg(0, 7);
>> -		set_debugreg(vcpu->arch.host_db[0], 0);
>> -		set_debugreg(vcpu->arch.host_db[1], 1);
>> -		set_debugreg(vcpu->arch.host_db[2], 2);
>> -		set_debugreg(vcpu->arch.host_db[3], 3);
>> +	if (unlikely(vcpu->arch.switch_db_regs || test_thread_flag(TIF_DEBUG))) {
>>      
> IIRC, you must clear dr7 before loading dr0..3 to avoid spurious effects.
>    

Yeah, I thought of it but forgot to implement it.

>> +		set_debugreg(current->thread.debugreg0, 0);
>> +		set_debugreg(current->thread.debugreg1, 1);
>> +		set_debugreg(current->thread.debugreg2, 2);
>> +		set_debugreg(current->thread.debugreg3, 3);
>> +		set_debugreg(current->thread.debugreg6, 6);
>> +		set_debugreg(current->thread.debugreg7, 7);
>>   	}
>> -	set_debugreg(vcpu->arch.host_dr6, 6);
>> -	set_debugreg(vcpu->arch.host_dr7, 7);
>>      
> Unless I miss something ATM, moving this into the if-clause should cause
> troubles if the guest leaves dr7 armed behind. But I need to refresh my
> memory on this.
>    

The hardware will clear dr7 on exit.

Updated patch to follow.

-- 
error compiling committee.c: too many arguments to function