linux-kernel - Re: [PATCH v6 8/9] KVM: VMX: Open code VMX preemption timer rate mask in its accessor

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Date: Fri, 15 Mar 2024 10:54:14 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Zhao Liu <zhao1.liu@...el.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	Paolo Bonzini <pbonzini@...hat.com>, Andy Lutomirski <luto@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	Shan Kang <shan.kang@...el.com>, Kai Huang <kai.huang@...el.com>, Xin Li <xin3.li@...el.com>
Subject: Re: [PATCH v6 8/9] KVM: VMX: Open code VMX preemption timer rate mask
 in its accessor

On Fri, Mar 15, 2024, Zhao Liu wrote:
> On Fri, Mar 08, 2024 at 05:27:24PM -0800, Sean Christopherson wrote:
> > Use vmx_misc_preemption_timer_rate() to get the rate in hardware_setup(),
> > and open code the rate's bitmask in vmx_misc_preemption_timer_rate() so
> > that the function looks like all the helpers that grab values from
> > VMX_BASIC and VMX_MISC MSR values.

..

> > -#define VMX_MISC_PREEMPTION_TIMER_RATE_MASK	GENMASK_ULL(4, 0)
> >  #define VMX_MISC_SAVE_EFER_LMA			BIT_ULL(5)
> >  #define VMX_MISC_ACTIVITY_HLT			BIT_ULL(6)
> >  #define VMX_MISC_ACTIVITY_SHUTDOWN		BIT_ULL(7)
> > @@ -162,7 +161,7 @@ static inline u32 vmx_basic_vmcs_mem_type(u64 vmx_basic)
> >  
> >  static inline int vmx_misc_preemption_timer_rate(u64 vmx_misc)
> >  {
> > -	return vmx_misc & VMX_MISC_PREEMPTION_TIMER_RATE_MASK;
> > +	return vmx_misc & GENMASK_ULL(4, 0);
> >  }
> 
> I feel keeping VMX_MISC_PREEMPTION_TIMER_RATE_MASK is clearer than
> GENMASK_ULL(4, 0), and the former improves code readability.
> 
> May not need to drop VMX_MISC_PREEMPTION_TIMER_RATE_MASK?

I don't necessarily disagree, but in this case I value consistency over one
individual case.  As called out in the changelog, the motivation is to make
vmx_misc_preemption_timer_rate() look like all the surrounding helpers.

_If_ we want to preserve the mask, then we should add #defines for vmx_misc_cr3_count(),
vmx_misc_max_msr(), etc.

I don't have a super strong preference, though I think my vote would be to not
add the masks and go with this patch.  These helpers are intended to be the _only_
way to access the fields, i.e. they effectively _are_ the mask macros, just in
function form.