Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 18 Oct 2022 11:13:50 -0700
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     "Li, Xin3" <xin3.li@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>
CC:     "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "brgerst@gmail.com" <brgerst@gmail.com>,
        "Bae, Chang Seok" <chang.seok.bae@intel.com>
Subject: =?US-ASCII?Q?RE=3A_=5BPATCH_v3_4/6=5D_x86/gsseg=3A_move_local=5Fi?= =?US-ASCII?Q?rq=5Fsave/restore=28=29_into_asm=5Fload=5Fgs=5Findex=28=29?=
User-Agent: K-9 Mail for Android
In-Reply-To: <BN6PR1101MB2161B983B1A85CE3803A3EF8A8289@BN6PR1101MB2161.namprd11.prod.outlook.com>
References: <20221013200134.1487-1-xin3.li@intel.com> <20221013200134.1487-5-xin3.li@intel.com> <8735bpbhat.ffs@tglx> <BN6PR1101MB2161B983B1A85CE3803A3EF8A8289@BN6PR1101MB2161.namprd11.prod.outlook.com>
Message-ID: <629F4B18-CCA4-4409-946E-F00146C6BCB5@zytor.com>
MIME-Version: 1.0
Content-Type: text/plain;
 charset=utf-8
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On October 18, 2022 10:25:31 AM PDT, "Li, Xin3" <xin3=2Eli@intel=2Ecom> wro=
te:
>> >  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);
>> >  }
>>=20
>> static inline void native_load_gs_index(unsigned int selector) {
>> 	unsigned long flags;
>>=20
>>         if (cpu_feature_enabled(LKGS)) {
>>               native_lkgs(selector);
>>         } else {
>> 	      local_irq_save(flags);
>>               asm_load_gs_index(selector);
>> 	      local_irq_restore(flags);
>>        }
>> }
>>=20
>> For paravirt enabled kernels we want during feature detection:
>>=20
>>         if (cpu_feature_enabled(LKGS)))
>>         	pv_ops=2Ecpu=2Eload_gs_index =3D 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=2Ecpu=2Eload_gs_index, right?
>
>Thanks!
>Xin
>
>>=20
>> No?
>>=20
>> Thanks,
>>=20
>>         tglx
>

You don't *have* to, but it would mean a branch to a branch, so it would b=
e more efficient=2E It would strictly be an optimization=2E