From: "Li, Xin3" <xin3.li@intel.com>
To: "H. Peter Anvin" <hpa@zytor.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: RE: [PATCH v3 4/6] x86/gsseg: move local_irq_save/restore() into asm_load_gs_index()
Date: Tue, 18 Oct 2022 21:29:14 +0000 [thread overview]
Message-ID: <BN6PR1101MB216107B32F33A18605FD3869A8289@BN6PR1101MB2161.namprd11.prod.outlook.com> (raw)
In-Reply-To: <629F4B18-CCA4-4409-946E-F00146C6BCB5@zytor.com>
> On October 18, 2022 10:25:31 AM PDT, "Li, Xin3" <xin3.li@intel.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.
Right, the generated object file shows that static_cpu_has is less efficient than ALTERNATIVE.
next prev parent reply other threads:[~2022-10-18 21:29 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-13 20:01 [PATCH v3 0/6] Enable LKGS instruction Xin Li
2022-10-13 20:01 ` [PATCH v3 1/6] x86/cpufeature: add cpu feature bit for LKGS Xin Li
2022-10-13 20:01 ` [PATCH v3 2/6] x86/opcode: add LKGS instruction to x86-opcode-map Xin Li
2022-10-13 20:01 ` [PATCH v3 3/6] x86/gsseg: make asm_load_gs_index() take an u16 Xin Li
2022-10-14 12:28 ` David Laight
2022-10-15 0:13 ` Li, Xin3
2022-10-15 2:41 ` H. Peter Anvin
2022-10-17 7:49 ` David Laight
2022-10-17 18:39 ` H. Peter Anvin
2022-10-13 20:01 ` [PATCH v3 4/6] x86/gsseg: move local_irq_save/restore() into asm_load_gs_index() Xin Li
2022-10-15 8:51 ` Thomas Gleixner
2022-10-18 17:25 ` Li, Xin3
2022-10-18 18:13 ` H. Peter Anvin
2022-10-18 21:29 ` Li, Xin3 [this message]
2022-10-13 20:01 ` [PATCH v3 5/6] x86/gsseg: move load_gs_index() to its own header file Xin Li
2022-10-13 20:01 ` [PATCH v3 6/6] x86/gsseg: use the LKGS instruction if available for load_gs_index() Xin Li
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