Messages in this thread | | | Date | Mon, 25 Nov 2019 08:13:48 -0800 | From | Sean Christopherson <> | Subject | Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by kernel parameter |
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On Fri, Nov 22, 2019 at 04:30:56PM -0800, Luck, Tony wrote: > On Fri, Nov 22, 2019 at 04:27:15PM +0100, Peter Zijlstra wrote: > > This all looks dubious on an HT system .... three snips > from your patch: > > > +static bool __sld_msr_set(bool on) > > +{ > > + u64 test_ctrl_val; > > + > > + if (rdmsrl_safe(MSR_TEST_CTRL, test_ctrl_val)) > > + return false; > > + > > + if (on) > > + test_ctrl_val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT; > > + else > > + test_ctrl_val &= ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT; > > + > > + if (wrmsrl_safe(MSR_TEST_CTRL, test_ctrl_val)) > > + return false; > > + > > + return true; > > +} > > > +void switch_sld(struct task_struct *prev) > > +{ > > + __sld_set_msr(true); > > + clear_tsk_thread_flag(current, TIF_CLD); > > +} > > > @@ -654,6 +654,9 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p) > > /* Enforce MSR update to ensure consistent state */ > > __speculation_ctrl_update(~tifn, tifn); > > } > > + > > + if (tifp & _TIF_SLD) > > + switch_sld(prev_p); > > } > > Don't you have some horrible races between the two logical > processors on the same core as they both try to set/clear the > MSR that is shared at the core level?
Yes and no. Yes, there will be races, but they won't be fatal in any way.
- Only the split-lock bit is supported by the kernel, so there isn't a risk of corrupting other bits as both threads will rewrite the current hardware value.
- Toggling of split-lock is only done in "warn" mode. Worst case scenario of a race is that a misbehaving task will generate multiple #AC exceptions on the same instruction. And this race will only occur if both siblings are running tasks that generate split-lock #ACs, e.g. a race where sibling threads are writing different values will only occur if CPUx is disabling split-lock after an #AC and CPUy is re-enabling split-lock after *its* previous task generated an #AC.
- Transitioning between modes at runtime isn't supported and disabling is tracked per task, so hardware will always reach a steady state that matches the configured mode. I.e. split-lock is guaranteed to be enabled in hardware once all _TIF_SLD threads have been scheduled out.
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