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

Jan Kiszka
2009-09-01T12:32:22Z

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

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