From: Peter Zijlstra <peterz@infradead.org>
To: "Woodhouse, David" <dwmw@amazon.co.uk>
Cc: "andi@firstfloor.org" <andi@firstfloor.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"tim.c.chen@linux.intel.com" <tim.c.chen@linux.intel.com>,
"tglx@linutronix.de" <tglx@linutronix.de>,
"ak@linux.intel.com" <ak@linux.intel.com>,
"pjt@google.com" <pjt@google.com>,
"dave.hansen@intel.com" <dave.hansen@intel.com>,
"luto@amacapital.net" <luto@amacapital.net>,
"gregkh@linux-foundation.org" <gregkh@linux-foundation.org>
Subject: Re: [PATCH] x86/retpoline: Avoid return buffer underflows on context switch
Date: Mon, 8 Jan 2018 23:26:04 +0100 [thread overview]
Message-ID: <20180108222604.GW29822@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <1515449839.4423.37.camel@amazon.co.uk>
On Mon, Jan 08, 2018 at 10:17:19PM +0000, Woodhouse, David wrote:
> On Mon, 2018-01-08 at 23:11 +0100, Peter Zijlstra wrote:
> >
> > So pjt did alignment, a single unroll and per discussion earlier today
> > (CET) or late last night (PST), he only does 16.
>
> Hey Intel, please tell us precisely how many RSB entries there are, on
> each family of CPU... :)
Right, and we can always fall back to 32 for unknown models.
> > Also, pause is unlikely to stop speculation, that comment doesn't make
> > sense. Looking at PJT's version there used to be a speculation trap in
> > there, but I can't see that here.
>
> In this particular code we don't need a speculation trap; that's
> elsewhere. This one is *just* about the call stack. And the reason we
> don't just have...
>
> call . + 5
> call . + 5
> call . + 5
> ...
>
> is because that might get interpreted as a "push %rip" and not go on
> the RSB at all. Hence the 'pause' between each one.
OK, then make the comment say that.
next prev parent reply other threads:[~2018-01-08 22:26 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-08 20:15 [PATCH] x86/retpoline: Avoid return buffer underflows on context switch Andi Kleen
2018-01-08 21:38 ` David Woodhouse
2018-01-08 22:54 ` Andi Kleen
2018-01-08 22:11 ` Peter Zijlstra
2018-01-08 22:17 ` Woodhouse, David
2018-01-08 22:26 ` Peter Zijlstra [this message]
2018-01-08 23:25 ` Woodhouse, David
2018-01-08 22:25 ` Andi Kleen
2018-01-08 22:58 ` Andi Kleen
2018-01-09 0:15 ` Paul Turner
2018-01-09 0:15 ` Paul Turner
2018-01-09 0:44 ` Woodhouse, David
2018-01-09 0:48 ` David Woodhouse
2018-01-09 2:48 ` Paul Turner
2018-01-09 3:05 ` David Woodhouse
2018-01-12 11:15 ` David Laight
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