Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 25 Jan 2019 16:39:09 +0100 (CET)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Zhenzhong Duan <zhenzhong.duan@oracle.com>
cc:     linux-kernel@vger.kernel.org, mingo@redhat.com,
        konrad.wilk@oracle.com, x86@kernel.org, srinivas.eeda@oracle.com,
        bp@suse.de, tim.c.chen@linux.intel.com, peterz@infradead.org,
        hpa@zytor.com
Subject: Re: [PATCH] x86/speculation: Update TIF_SPEC_IB before ibpb
 barrier
In-Reply-To: <alpine.DEB.2.21.1901231335570.1628@nanos.tec.linutronix.de>
Message-ID: <alpine.DEB.2.21.1901251637480.1622@nanos.tec.linutronix.de>
References: <48a105d3-fa32-40e4-9775-37d49f42eac0@default> <alpine.DEB.2.21.1901231335570.1628@nanos.tec.linutronix.de>
User-Agent: Alpine 2.21 (DEB 202 2017-01-01)
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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