linux-kernel - Re: [RESEND PATCH 1/2] KVM: X86: Add support for the emulation of DR6_BUS

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Date:   Thu, 28 Jan 2021 08:25:16 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Chenyi Qiang <chenyi.qiang@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH 1/2] KVM: X86: Add support for the emulation of
 DR6_BUS_LOCK bit

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.

Paolo