linux-kernel - RE: [PATCH v12 10/18] x86/fsgsbase/64: Enable FSGSBASE instructions in helper functions

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Date:   Wed, 20 May 2020 09:13:14 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Thomas Gleixner' <tglx@...utronix.de>,
        'Sasha Levin' <sashal@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "bp@...en8.de" <bp@...en8.de>, "luto@...nel.org" <luto@...nel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "ravi.v.shankar@...el.com" <ravi.v.shankar@...el.com>,
        "chang.seok.bae@...el.com" <chang.seok.bae@...el.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: RE: [PATCH v12 10/18] x86/fsgsbase/64: Enable FSGSBASE instructions
 in helper functions

From: Thomas Gleixner
> Sent: 19 May 2020 15:48
> 
> David Laight <David.Laight@...LAB.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

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