Re: [PATCH] KVM: VMX: Conditionally reload debug register 6

[Jan Kiszka <jan.kiszka@siemens.com>]

Avi Kivity wrote:
> On 09/01/2009 02:43 PM, Jan Kiszka wrote:
>> @@ -3731,7 +3732,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>>>   				| (1<<  VCPU_EXREG_PDPTR));
>>>   	vcpu->arch.regs_dirty = 0;
>>>
>>> -	get_debugreg(vcpu->arch.dr6, 6);
>>> +	if (vcpu->arch.switch_db_regs)
>>> +		get_debugreg(vcpu->arch.dr6, 6);
>>>
>>>   	vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD);
>>>   	if (vmx->rmode.irq.pending)
>>>      
>> That reduces the emulation quality as vcpu->arch.switch_db_regs is only
>> set if some breakpoint is active while dr6 has its use also when that is
>> not the case).
>>    
> 
> True - there's the TF reason reporting bits.
> 
> How about this then:
> 
> - if !switch_db_regs, trap #DB
> - on #DB trap, copy DR6.BS and DR6.BT to vcpu->arch.dr6, and reinject 
> the #DB
> 
> ?

I'm worried about vm-exits that may take precedence over the #db trap.
If we skip to save/restore dr6 for them, the value that the interception
handler sees later on will be bogus. Or is this architecturally impossible?

Jan

-- 
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux