linux-kernel - Re: [PATCH 3/7] x86/enter: Use IBRS on syscall and interrupts

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Date:   Thu, 4 Jan 2018 20:51:56 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/7] x86/enter: Use IBRS on syscall and interrupts

On Thu, Jan 4, 2018 at 4:08 PM, Dave Hansen <dave.hansen@...el.com> wrote:
> On 01/04/2018 02:33 PM, Peter Zijlstra wrote:
>> On Thu, Jan 04, 2018 at 09:56:44AM -0800, Tim Chen wrote:
>>> Set IBRS upon kernel entrance via syscall and interrupts. Clear it
>>> upon exit.
>>
>> So not only did we add a CR3 write, we're now adding an MSR write to the
>> entry/exit paths. Please tell me that these are 'fast' MSRs? Given
>> people are already reporting stupid numbers with just the existing
>> PTI/CR3, what kind of pain are we going to get from adding this?
>
> This "dynamic IBRS" that does runtime switching will not be on by
> default and will be patched around by alternatives unless someone
> explicitly opts in.
>
> If you decide you want the additional protection that it provides, you
> can take the performance hit.  How much is that?  We've been saying that
> these new MSRs are roughly as expensive as the CR3 writes.  How
> expensive are those?  Don't take my word for it, a few folks were
> talking about it today:
>
> Google says[1]: "We see negligible impact on performance."
> Amazon says[2]: "We don’t expect meaningful performance impact."
>
> I chopped a few qualifiers out of there, but I think that roughly
> captures the sentiment.
>
> 1.
> https://security.googleblog.com/2018/01/more-details-about-mitigations-for-cpu_4.html
> 2.
> http://www.businessinsider.com/google-amazon-performance-hit-meltdown-spectre-fixes-overblown-2018-1

Do we need an arch_prctl() to enable IBRS for user mode?