linux-kernel - Re: [PATCH v2 1/2] x86/traps: Initialize DR6 by writing its architectural reset value

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Message-ID: <c8b0924e-4cbf-473b-99e2-eb212d8e99bf@zytor.com>
Date: Tue, 17 Jun 2025 17:42:51 -0700
From: Xin Li <xin@...or.com>
To: Sohil Mehta <sohil.mehta@...el.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, pbonzini@...hat.com,
        peterz@...radead.org, brgerst@...il.com, tony.luck@...el.com,
        fenghuay@...dia.com
Subject: Re: [PATCH v2 1/2] x86/traps: Initialize DR6 by writing its
 architectural reset value

On 6/17/2025 4:10 PM, Sohil Mehta wrote:
>> Ah, and now I see that DR6_RESERVED is an existing #define in a uAPI header (Xin
>> said there were a few, but I somehow missed them earlier).  Maybe just leave that
>> thing alone, but update the comment to state that it's a historical wart?  And
>> then put DR6_ACTIVE_LOW and other macros in arch/x86/include/asm/debugreg.h?
>>
> Yeah, that's unfortunate. Updating the comment seems the best we do for now.

It's a mess, and I don't think I can just remove DR6_RESERVED.

But the value 0xFFFF0FF0 no longer makes sense on newer CPUs, so maybe 
it won't cause any real problem?