linux-kernel - RE: [PATCH v3 4/6] x86/gsseg: move local_irq_save/restore() into asm_load_gs

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Date:   Tue, 18 Oct 2022 11:13:50 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     "Li, Xin3" <xin3.li@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "brgerst@...il.com" <brgerst@...il.com>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>
Subject: RE: [PATCH v3 4/6] x86/gsseg: move local_irq_save/restore() into asm_load_gs_index()

On October 18, 2022 10:25:31 AM PDT, "Li, Xin3" <xin3.li@...el.com> wrote:
>> >  static inline void native_load_gs_index(unsigned int selector)  {
>> > -	unsigned long flags;
>> > -
>> > -	local_irq_save(flags);
>> >  	asm_load_gs_index(selector);
>> > -	local_irq_restore(flags);
>> >  }
>> 
>> static inline void native_load_gs_index(unsigned int selector) {
>> 	unsigned long flags;
>> 
>>         if (cpu_feature_enabled(LKGS)) {
>>               native_lkgs(selector);
>>         } else {
>> 	      local_irq_save(flags);
>>               asm_load_gs_index(selector);
>> 	      local_irq_restore(flags);
>>        }
>> }
>> 
>> For paravirt enabled kernels we want during feature detection:
>> 
>>         if (cpu_feature_enabled(LKGS)))
>>         	pv_ops.cpu.load_gs_index = native_lkgs;
>
>If we use static_cpu_has in native_load_gs_index
>       if (static_cpu_has(X86_FEATURE_LKGS)) {
>               native_lkgs(selector);
>       }
>
>We don't have to change pv_ops.cpu.load_gs_index, right?
>
>Thanks!
>Xin
>
>> 
>> No?
>> 
>> Thanks,
>> 
>>         tglx
>

You don't *have* to, but it would mean a branch to a branch, so it would be more efficient. It would strictly be an optimization.