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

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Message-ID: <1e063b73-0f9a-4956-9634-2552e6e63ee1@intel.com>
Date: Mon, 1 Apr 2024 15:07:13 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>, 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 3/16/2024 1:54 AM, Sean Christopherson wrote:
> 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.
> 

+1.

However, it seems different for vmx_basic_vmcs_mem_type() in patch 5, 
that I just recommended to define the MASK.

Because we already have

	#define VMX_BASIC_MEM_TYPE_SHIFT	50

and it has been used in vmx/nested.c,

static inline u32 vmx_basic_vmcs_mem_type(u64 vmx_basic)
{
	return (vmx_basic & GENMASK_ULL(53, 50)) >>
		VMX_BASIC_MEM_TYPE_SHIFT;
}

looks not intuitive than original patch.