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Reply-To: zhenzhong.duan@oracle.com
Subject: Re: [PATCH] x86/speculation: Update TIF_SPEC_IB before ibpb barrier
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     linux-kernel@vger.kernel.org, mingo@redhat.com,
        konrad.wilk@oracle.com, x86@kernel.org, srinivas.eeda@oracle.com,
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From:   Zhenzhong Duan <zhenzhong.duan@oracle.com>
Organization: Oracle
Message-ID: <26c4fb45-eaa1-9520-bd35-1cf8ca673e30@oracle.com>
Date:   Mon, 28 Jan 2019 16:42:41 +0800
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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