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

[Xin Li]

On 6/13/2025 7:10 AM, Sean Christopherson wrote:
> 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.

I guess I was too aggressive to make overkill changes in a bug-fixing
patch, which will be back-ported.

I will revise the patch set to focus on bug fixing first.

> 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 :-)

I did think about changing DR6_ to DR_STATUS_ and DR7_ to DR_CONTROL_ ;)

However it seems that DR7_ and DR6_ are de facto prefixes in the kernel
code, and everyone appears to recognize their intended use.  It's better
for me to leave them as-is.

> #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