linux-kernel - Re: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC

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Date:   Sat, 6 Jan 2018 10:20:26 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Konrad Rzeszutek Wilk <konrad@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to enable/disable
 SPEC_CTRL feature



On 01/06/2018 06:41 AM, Konrad Rzeszutek Wilk wrote:
> On Fri, Jan 05, 2018 at 06:12:19PM -0800, Tim Chen wrote:
>> From: Tim Chen <tim.c.chen@...ux.intel.com>
>> From: Andrea Arcangeli <aarcange@...hat.com>
>>
>
>>  
>>  .macro DISABLE_IBRS
>> -	ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_SPEC_CTRL
>> +	testl	$1, dynamic_ibrs
> 
> On every system call we end up hammering on this 'dynamic_ibrs'
> variable. And it looks like it can be flipped via the IPI mechanism.

On system call, we read dynamic_ibrs value (not change it) and
flip the IBRS msr only if it dynamic_ibrs is true.

We only do global change to all the IBRS msrs on all cpus during
the admin request to change its value, serialized by
spec_ctrl_mutex.  Before we do that, we set dynamic_ibrs to 0,
so each cpu no longer do any change to IBRS.  Then the IPI happens
to update the IBRS MSR values.

> 
> Would it make sense for this to be per-cpu?
>