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

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Date: Thu, 25 Apr 2024 22:18:02 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Zhao Liu <zhao1.liu@...el.com>, 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 4/25/2024 4:06 AM, Sean Christopherson wrote:
> On Tue, Apr 02, 2024, Sean Christopherson wrote:
>> On Mon, Apr 01, 2024, Xiaoyao Li wrote:
>>> 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.
>>
>> Yeah, agreed, that's taking the worst of both worlds.  I'll update patch 5 to drop
>> VMX_BASIC_MEM_TYPE_SHIFT when effectively "moving" it into vmx_basic_vmcs_mem_type().
> 
> Drat.  Finally getting back to this, dropping VMX_BASIC_MEM_TYPE_SHIFT doesn't
> work because it's used by nested_vmx_setup_basic(), as is VMX_BASIC_VMCS_SIZE_SHIFT,
> which is presumably why past me kept them around.
> 
> I'm leaning towards keeping things as proposed in this series.  

If it goes this way, I beg for a comment above the code to explain. 
Otherwise, people might send patch to use defined MARCO in the future.

> I don't see us
> gaining a third copy, or even a third user, i.e. I don't think we are creating a
> future problem by open coding the shift in vmx_basic_vmcs_mem_type().  And IMO
> code like this
> 
> 	return (vmx_basic & VMX_BASIC_MEM_TYPE_MASK) >>
> 	       VMX_BASIC_MEM_TYPE_SHIFT;
> 
> is an unnecessary obfuscation when there is literally one user (the accessor).
> 
> Another idea would be to delete VMX_BASIC_MEM_TYPE_SHIFT and VMX_BASIC_VMCS_SIZE_SHIFT,
> and either open code the values or use local const variables, but that also seems
> like a net negative, e.g. splits the effective definitions over too many locations.