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 15:17:19 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.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 1/27/2021 6:04 PM, Paolo Bonzini wrote:
> On 27/01/21 04:41, Xiaoyao Li wrote:
>> On 1/27/2021 12:31 AM, Paolo Bonzini wrote:
>>> On 08/01/21 07:49, Chenyi Qiang wrote:
>>>> To avoid breaking the CPUs without bus lock detection, activate the
>>>> DR6_BUS_LOCK bit (bit 11) conditionally in DR6_FIXED_1 bits.
>>>>
>>>> The set/clear of DR6_BUS_LOCK is similar to the DR6_RTM in DR6
>>>> register. The processor clears DR6_BUS_LOCK when bus lock debug
>>>> exception is generated. (For all other #DB the processor sets this bit
>>>> to 1.) Software #DB handler should set this bit before returning to the
>>>> interrupted task.
>>>>
>>>> For VM exit caused by debug exception, bit 11 of the exit qualification
>>>> is set to indicate that a bus lock debug exception condition was
>>>> detected. The VMM should emulate the exception by clearing bit 11 of 
>>>> the
>>>> guest DR6.
>>>
>>> Please rename DR6_INIT to DR6_ACTIVE_LOW, and then a lot of changes 
>>> become simpler:
>>
>> Paolo,
>>
>> What do you want to convey with the new name DR6_ACTIVE_LOW? To be 
>> honest, the new name is confusing to me.
> 
> "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.

> Paolo
> 
>>>> -        dr6 |= DR6_BD | DR6_RTM;
>>>> +        dr6 |= DR6_BD | DR6_RTM | DR6_BUS_LOCK;
>>>
>>> dr6 |= DR6_BD | DR6_ACTIVE_LOW;
>>>
>>
>>
>