linux-kernel - Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec

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Message-ID: <20180104014100.3786e686@alans-desktop>
Date:   Thu, 4 Jan 2018 01:41:00 +0000
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     Jiri Kosina <jikos@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arch@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Elena Reshetova <elena.reshetova@...el.com>
Subject: Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier

On Thu, 4 Jan 2018 02:27:54 +0100 (CET)
Jiri Kosina <jikos@...nel.org> wrote:

> On Thu, 4 Jan 2018, Alan Cox wrote:
> 
> > There are people trying to tune coverity and other tool rules to identify
> > cases,   
> 
> Yeah, but that (and *especially* Coverity) is so inconvenient to be 
> applied to each and every patch ... that this is not the way to go.

Agreed enitely - and coverity is non-free which is even worse as a
dependancy. Right now we are in the 'what could be done quickly by a few
people' space. The papers are now published, so the world can work on
better solutions and extending more convenient tooling.

> If the CPU speculation can cause these kinds of side-effects, it just must 
> not happen, full stop. 

At which point your performance will resemble that of a 2012 atom
processor at best.

> OS trying to work it around is just a whack-a-mole 
> (which we can apply for old silicon, sure ... but not something that 
> becomes a new standard) that's never going to lead to any ultimate 
> solution.

In the ideal world it becomes possible for future processors to resolve
such markings down to no-ops. Will that be possible or will we get more
explicit ways to tell the processor what is unsafe - I don't
personally know but I do know that turning off speculation is not the
answer.

Clearly if the CPU must be told then C is going to have to grow some
syntax for it and some other languages are going to see 'taint' moving
from a purely software construct to a real processor one.

Alan