linux-kernel - Re: [PATCH 6/7] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC

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Date:   Fri, 5 Jan 2018 17:29:27 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     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>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/7] x86/spec_ctrl: Add sysctl knobs to enable/disable
 SPEC_CTRL feature

On 01/05/2018 03:16 AM, David Woodhouse wrote:
> On Thu, 2018-01-04 at 09:56 -0800, Tim Chen wrote:
>>
>> +       mutex_lock(&spec_ctrl_mutex);
>> +
>> +       if (enable == IBRS_DISABLED) {
>> +               /* disable IBRS usage */
>> +               set_ibrs_disabled();
>> +               if (spec_ctrl_ibrs & SPEC_CTRL_IBRS_SUPPORTED)
>> +                       spec_ctrl_flush_all_cpus(MSR_IA32_SPEC_CTRL, SPEC_CTRL_FEATURE_DISABLE_IBRS);
>> +       } else if (enable == IBRS_ENABLED) {
>> +               /* enable IBRS usage in kernel */
>> +               clear_ibrs_disabled();
>> +               if (spec_ctrl_ibrs & SPEC_CTRL_IBRS_SUPPORTED)
>> +                       set_ibrs_inuse();
>> +               else
>> +                       /* Platform don't support IBRS */
>> +                       enable = IBRS_DISABLED;
>> +       } else if (enable == IBRS_ENABLED_USER) {
>> +               /* enable IBRS usage in both userspace and kernel */
>> +               clear_ibrs_disabled();
>> +               /* don't change IBRS value once we set it to always on */
>> +               clear_ibrs_inuse();
>> +               if (spec_ctrl_ibrs & SPEC_CTRL_IBRS_SUPPORTED)
>> +                       spec_ctrl_flush_all_cpus(MSR_IA32_SPEC_CTRL, SPEC_CTRL_FEATURE_ENABLE_IBRS);
>> +               else
>> +                       /* Platform don't support IBRS */
>> +                       enable = IBRS_DISABLED;
>> +       }
> 
> This doesn't take the retpoline status into account. If we have
> retpoline, we don't need IBRS in the kernel.
> 

If retpoline is used, we don't enable IBRS automatically during feature detection.
But if the admin is paranoid, he still has the choice to explicitly issue
a command to enable IBRS here.

Tim