Messages in this thread | | | Date | Mon, 28 Jan 2019 09:36:51 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH] x86/speculation: Update TIF_SPEC_IB before ibpb barrier |
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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.
Thanks,
tglx
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