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

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Message-ID: <276d66a1-dafe-4ed8-aefd-8f60e34ff575@intel.com>
Date: Tue, 17 Jun 2025 16:10:26 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: "Xin Li (Intel)" <xin@...or.com>, <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 1:47 PM, Sean Christopherson 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.

> /*
>  * DR6_ACTIVE_LOW combines fixed-1 and active-low bits.
>  * We can regard all the bits in DR6_FIXED_1 as active_low bits;
>  * they will never be 0 for now, but when they are defined
>  * in the future it will require no code change.
>  *
>  * DR6_ACTIVE_LOW is also used as the init/reset value for DR6.
>  */
> #define DR6_ACTIVE_LOW	0xffff0ff0
> #define DR6_VOLATILE	0x0001e80f
> #define DR6_FIXED_1	(DR6_ACTIVE_LOW & ~DR6_VOLATILE)