Re: [RESEND PATCH 1/2] KVM: X86: Add support for the emulation of DR6_BUS_LOCK bit

Xiaoyao Li
2021-01-28T07:44:14Z

On 1/28/2021 3:25 PM, Paolo Bonzini wrote:
> On 28/01/21 08:17, Xiaoyao Li wrote:
>>>
>>> "Active low" means that the bit is usually 1 and goes to 0 when the
>>> condition (such as RTM or bus lock) happens.  For almost all those
>>> DR6 bits the value is in fact always 1, but if they are defined in
>>> the future it will require no code change.
>>
>> Why not keep use DR6_INIT, or DR6_RESET_VALUE? or any other better name.
>>
>> It's just the default clear value of DR6 that no debug condition is hit.
>
> I preferred "DR6_ACTIVE_LOW" because the value is used only once or
> twice to initialize dr6, and many times to invert those bits.  For example:
>
> vcpu->arch.dr6 &= ~DR_TRAP_BITS;
> vcpu->arch.dr6 |= DR6_ACTIVE_LOW;
> vcpu->arch.dr6 |= payload;
> vcpu->arch.dr6 ^= payload & DR6_ACTIVE_LOW;
>
> payload = vcpu->arch.dr6;
> payload &= ~DR6_BT;
> payload ^= DR6_ACTIVE_LOW;
>
> The name conveys that it's not just the initialization value; it's also
> the XOR mask between the #DB exit qualification (which we also use as
> the "payload") and DR6.

Fair enough.