linux-kernel - Re: [PATCH] x86/speculation: Update TIF_SPEC

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Date:   Fri, 25 Jan 2019 16:39:09 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Zhenzhong Duan <zhenzhong.duan@...cle.com>
cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        konrad.wilk@...cle.com, x86@...nel.org, srinivas.eeda@...cle.com,
        bp@...e.de, tim.c.chen@...ux.intel.com, peterz@...radead.org,
        hpa@...or.com
Subject: Re: [PATCH] x86/speculation: Update TIF_SPEC_IB before ibpb
 barrier

On Wed, 23 Jan 2019, Thomas Gleixner wrote:

> On Fri, 18 Jan 2019, Zhenzhong Duan wrote:
> 
> > When a task is set for updating TIF_SPEC_IB throuth SECCOMP by others
> > and it's scheduled in the first time, a stale TIF_SPEC_IB value is
> > picked in cond_ibpb(). This is due to TIF_SPEC_IB is updated later at
> > __switch_to_xtra().
> > 
> > Add an extra call to speculation_ctrl_update_tif() to update it before
> > IBPB barrier.
> 
> Errm. No. It adds that call to speculation_ctrl_update_tif() for every
> mm switch, most of the time for nothing.
> 
> If at all, and we discussed that before and decided not to worry about it
> (because it gets fixed up on the next context switch), then you want to
> handle ibpb() from there:

Actually we need to do that. It's not only the scheduled in first
problem. That whole thing might become completely stale in either
direction. Care to whip up a patch?

Thanks,

	tglx