Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   David Laight <David.Laight@ACULAB.COM>
To:     'Thomas Gleixner' <tglx@linutronix.de>,
        'Sasha Levin' <sashal@kernel.org>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "bp@alien8.de" <bp@alien8.de>, "luto@kernel.org" <luto@kernel.org>,
        "hpa@zytor.com" <hpa@zytor.com>,
        "dave.hansen@intel.com" <dave.hansen@intel.com>,
        "tony.luck@intel.com" <tony.luck@intel.com>,
        "ak@linux.intel.com" <ak@linux.intel.com>,
        "ravi.v.shankar@intel.com" <ravi.v.shankar@intel.com>,
        "chang.seok.bae@intel.com" <chang.seok.bae@intel.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        "x86@kernel.org" <x86@kernel.org>
Subject: RE: [PATCH v12 10/18] x86/fsgsbase/64: Enable FSGSBASE instructions
 in helper functions
Thread-Topic: [PATCH v12 10/18] x86/fsgsbase/64: Enable FSGSBASE instructions
 in helper functions
Thread-Index: AQHWLVJcG1AiHrfpmEaP5P6pVvKjjqivU+WggAAZtICAAUUdcA==
Date:   Wed, 20 May 2020 09:13:14 +0000
Message-ID: <f35d0f8ddc2a4e518bd9674b74e40497@AcuMS.aculab.com>
References: <20200511045311.4785-11-sashal@kernel.org>
 <87v9ktw1ev.fsf@nanos.tec.linutronix.de> <20200518202435.GD33628@sasha-vm>
 <238a6e2609eb4e0497b13fbe5f531917@AcuMS.aculab.com>
 <87r1vg6kwq.fsf@nanos.tec.linutronix.de>
In-Reply-To: <87r1vg6kwq.fsf@nanos.tec.linutronix.de>
Accept-Language: en-GB, en-US
Content-Language: en-US
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8BIT
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

From: Thomas Gleixner
> Sent: 19 May 2020 15:48
> 
> David Laight <David.Laight@ACULAB.COM> writes:
> > From: Sasha Levin
> >> >> +		native_swapgs();
> >> >> +		gsbase = rdgsbase();
> >> >> +		native_swapgs();
> >
> > Does local_irq_save() even do anything useful here.
> > You need to actually execute CLI, not just set a
> > flag that indicates interrupts shouldn't happen.
> > (Which is what I think local_irq_save() might do.)
> 
>   local_irq_save()
>     raw_local_irq_save()
>       arch_local_irq_save()
>         arch_local_irq_disable()
>           native_irq_disable()
>             asm("CLI")

Ah, I was expecting software 'tricks' to avoid the expensive CLI.
But that call chain probably costs more - unless it is all inlined.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)