From: Peter Zijlstra <peterz@infradead.org>
To: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Andy Lutomirski <luto@kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Greg KH <gregkh@linuxfoundation.org>,
Dave Hansen <dave.hansen@intel.com>,
Andrea Arcangeli <aarcange@redhat.com>,
Andi Kleen <ak@linux.intel.com>,
Arjan Van De Ven <arjan.van.de.ven@intel.com>,
David Woodhouse <dwmw@amazon.co.uk>,
Dan Williams <dan.j.williams@intel.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Ashok Raj <ashok.raj@intel.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 3/5] x86/enter: Use IBRS on syscall and interrupts
Date: Wed, 10 Jan 2018 19:28:56 +0100 [thread overview]
Message-ID: <20180110182856.GQ6176@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <26d015ef-c5d4-b529-5c81-97115ec02f48@linux.intel.com>
On Wed, Jan 10, 2018 at 10:16:20AM -0800, Tim Chen wrote:
> On 01/10/2018 02:04 AM, Peter Zijlstra wrote:
> > What this Changelog fails to address is _WHY_ we need this. What does
> > this provide that retpoline does not.
> >
>
> Ok. I mentioned that in the cover letter that IBRS is a maximum security
> mode in the CPU itself to directly restrict all indirect branches to prevent SPECTRE v2.
>
> I'll also include such comments in the commit log here.
That still doesn't say anything useful. Why and where is it better than
retpoline? Why would I ever want to use IBRS? Those are not questions
that have clear answers here.
>From what I can gather of the discussion earlier today is that pre SKL
IBRS is no better than retpoline and a whole lot slower.
On SKL+ retpoline is mostly there, but has a few dinky holes in and it
_might_ make sense to use IBRS.
But I feel it needs explaining what the exact holes are (pjt and dwmw2
had a fair enumeration IIRC) such that people can judge the risk.
No wishy washy maybe nonsense, clear language.
next prev parent reply other threads:[~2018-01-10 18:29 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-10 2:26 [PATCH v3 0/5] IBRS patch series Tim Chen
2018-01-10 2:26 ` [PATCH v3 1/5] x86/feature: Detect the x86 IBRS feature to control Speculation Tim Chen
2018-01-10 2:26 ` [PATCH v3 2/5] x86/enter: Create macros to set/clear IBRS Tim Chen
2018-01-11 16:04 ` Thomas Gleixner
2018-01-11 21:05 ` Tim Chen
2018-01-11 21:24 ` Tim Chen
2018-01-10 2:26 ` [PATCH v3 3/5] x86/enter: Use IBRS on syscall and interrupts Tim Chen
2018-01-10 10:04 ` Peter Zijlstra
2018-01-10 11:27 ` Paolo Bonzini
2018-01-10 18:16 ` Tim Chen
2018-01-10 18:28 ` Peter Zijlstra [this message]
2018-01-10 18:44 ` Tim Chen
2018-01-10 18:47 ` Van De Ven, Arjan
2018-01-11 12:45 ` Woodhouse, David
2018-01-10 2:26 ` [PATCH v3 4/5] x86/ibrs: Create boot option for IBRS Tim Chen
2018-01-10 2:26 ` [PATCH v3 5/5] x86/idle: Disable IBRS entering idle and enable it on wakeup Tim Chen
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