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

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Date:   Mon, 28 Jan 2019 16:42:41 +0800
From:   Zhenzhong Duan <zhenzhong.duan@...cle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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 2019/1/28 16:36, Thomas Gleixner wrote:
> On Mon, 28 Jan 2019, Zhenzhong Duan wrote:
>> On 2019/1/26 2:03, Thomas Gleixner wrote:
>>> Bah, nonsense. Brain was clearly still out for lunch and I confused IBPB
>>> and STIBP for a moment. cond_ibpb() is the thing issues in switch_mm() and
>>> that is not leaving a stale MSR around because we only write to it when we
>>> need the barrier. The bit is not stale because the barrier is only issued
>>> with the write. The bit has not to be cleared.
>>>
>>> So the only 'issue' what happens is that switch_to() either issues a
>>> barrier too much or misses one. That's really not a problem.
>>
>> Ok, yes, the purpose of this patch is to avoid the one missed barrier.
> 
> And that missed barrier is not worth it to do extra work in switch_to/mm
> simply because it's a one off event and there is no way to exploit that
> reliably.

Got it.

Thanks
Zhenzhong