linux-kernel - Re: [PATCH v1 2/3] x86/traps: Initialize DR7 by writing its architectural reset value

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Message-ID: <aEwxcVzQubz3BmmJ@google.com>
Date: Fri, 13 Jun 2025 07:10:57 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
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, brgerst@...il.com, tony.luck@...el.com, 
	fenghuay@...dia.com
Subject: Re: [PATCH v1 2/3] x86/traps: Initialize DR7 by writing its
 architectural reset value

On Fri, Jun 13, 2025, Peter Zijlstra wrote:
> On Fri, Jun 13, 2025 at 12:01:16AM -0700, Xin Li (Intel) wrote:
> 
> > While at it, replace the hardcoded debug register number 7 with the
> > existing DR_CONTROL macro for clarity.
> 
> Yeah, not really a fan of that... IMO that obfuscates the code more than
> it helps, consider:

+1, and NAK to the KVM changes.  Pretty much everything in KVM deals with the
"raw" names.  The use of dr6 and dr7 is pervasive throughout the VMX and SVM
architectures:

 vmcs.GUEST_DR7
 vmcb.save.dr6
 vmcb.save.dr7

And is cemented in KVM's uAPI:

 kvm_debug_exit_arch.dr6
 kvm_debug_exit_arch.dr7
 kvm_debugregs.dr6
 kvm_debugregs.dr7

Using DR_STATUS and DR_CONTROL is not an improvement when everything else is using
'6' and '7'.  E.g. I skipped the changelog and was very confused by the '6' =>
DR_STATUS change in the next patch.

And don't even think about renaming the prefixes on these :-)

#define DR6_BUS_LOCK   (1 << 11)
#define DR6_BD		(1 << 13)
#define DR6_BS		(1 << 14)
#define DR6_BT		(1 << 15)
#define DR6_RTM		(1 << 16)
/*
 * 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)

#define DR7_BP_EN_MASK	0x000000ff
#define DR7_GE		(1 << 9)
#define DR7_GD		(1 << 13)
#define DR7_FIXED_1	0x00000400
#define DR7_VOLATILE	0xffff2bff