linux-kernel - Re: [PATCH 1/5] Add a global synchronization point for pvclock

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Date:	Tue, 20 Apr 2010 14:05:39 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Glauber Costa <glommer@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock

On 04/19/2010 11:39 PM, Avi Kivity wrote:
> On 04/19/2010 09:35 PM, Zachary Amsden wrote:
>>>>> Sockets and boards too?  (IOW, how reliable is TSC_RELIABLE)?
>>>> Not sure, IIRC we clear that when the TSC sync test fails, eg when we
>>>> mark the tsc clocksource unusable.
>>>
>>> Worrying.  By the time we detect this the guest may already have 
>>> gotten confused by clocks going backwards.
>>
>>
>> Upstream, we are marking the TSC unstable preemptively when hardware 
>> which will eventually sync test is detected, so this should be fine.
>
> ENOPARSE?
>

Instead of detecting TSC warp, c1e_idle, power_saving_mwait_init, 
tsc_check_state, dmi_mark_tsc_unstable all do something similar to this 
to disable TSC before warp even occurs:

static void c1e_idle(void)
{
         if (need_resched())
                 return;

         if (!c1e_detected) {
                 u32 lo, hi;

                 rdmsr(MSR_K8_INT_PENDING_MSG, lo, hi);
                 if (lo & K8_INTP_C1E_ACTIVE_MASK) {
                         c1e_detected = 1;
                         if (!boot_cpu_has(X86_FEATURE_NONSTOP_TSC))
                                 mark_tsc_unstable("TSC halt in AMD C1E");
                         printk(KERN_INFO "System has AMD C1E enabled\n");
                         set_cpu_cap(&boot_cpu_data, X86_FEATURE_AMDC1E);
                 }
         }


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