linux-kernel - Re: [PATCH 26/35] x86/enter: Create macros to stop/restart Indirect Branch Speculation

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Message-ID: <20180118194452.GA6849@linux.intel.com>
Date:   Thu, 18 Jan 2018 11:44:53 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Jason Baron <jbaron@...mai.com>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH 26/35] x86/enter: Create macros to stop/restart Indirect
 Branch Speculation

On Thu, Jan 18, 2018 at 02:48:26PM +0100, Peter Zijlstra wrote:
> From: Tim Chen <tim.c.chen@...ux.intel.com>
> 
> +
> +.macro STOP_IB_SPEC
> +	ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_IBRS

Peter,

Will it be possible to use static key here like
	STATIC_JUMP_IF_FALSE .Lskip_\@, specctrl_ibrs, def=0

In Thomas' original patchset, there is a specctrl_ibrs static key.

That will make a switch of static branch here easier if we want
to change it later in boot or at run time.

Tim 

> +	PUSH_MSR_REGS
> +	WRMSR_ASM $MSR_IA32_SPEC_CTRL, $0, $SPEC_CTRL_ENABLE_IBRS
> +	POP_MSR_REGS
> +.Lskip_\@:
> +.endm
> +