Re: [PATCH] x86: serialize LVTT and TSC_DEADLINE write

[Thomas Gleixner <tglx@linutronix.de>]

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On Sun, 2 Aug 2015, Shaohua Li wrote:

> On Sat, Aug 01, 2015 at 12:10:41PM +0200, Thomas Gleixner wrote:
> > On Fri, 31 Jul 2015, Shaohua Li wrote:
> > > @@ -336,6 +336,22 @@ static void __setup_APIC_LVTT(unsigned int clocks, int oneshot, int irqen)
> > >  	apic_write(APIC_LVTT, lvtt_value);
> > >  
> > >  	if (lvtt_value & APIC_LVT_TIMER_TSCDEADLINE) {
> > > +		u64 msr;
> > > +
> > > +		/*
> > > +		 * See Intel SDM: TSC-Deadline Mode chapter. In xAPIC mode,
> > > +		 * writing APIC LVTT and TSC_DEADLINE MSR isn't serialized.
> > > +		 * This uses the algorithm described in Intel SDM to serialize
> > > +		 * the two writes
> > > +		 * */
> > > +		while (1) {
> > > +			wrmsrl(MSR_IA32_TSC_DEADLINE, -1L);
> > > +			rdmsrl(MSR_IA32_TSC_DEADLINE, msr);
> > > +			if (msr)
> > > +				break;
> > > +		}
> > > +		wrmsrl(MSR_IA32_TSC_DEADLINE, 0);
> > 
> > 
> > I think this is exceptionally silly. A proper fence after the
> > apic_write() should have the same effect.
> 
> Not sure what happens in the hardware, I could have a try of fence, but
> I'd prefer using the algorithm Intel described. This is not a fast path,

s/algorithm/voodoo/

> the loop will exit immediately regardless the issue occurs anyway.

Well, the SDM also says:

 "To allow for efficient access to the APIC registers in x2APIC mode,
  the serializing semantics of WRMSR are relaxed when writing to the
  APIC registers. Thus, system software should not use “WRMSR to APIC
  registers in x2APIC mode” as a serializing instruction. Read and write
  accesses to the APIC registers will occur in program order. A WRMSR to
  an APIC register may complete before all preceding stores are globally
  visible; software can prevent this by inserting a serializing
  instruction, an SFENCE, or an MFENCE before the WRMSR."

And that's what happens here. The write to the LVT has not yet hit the
APIC, so the WRMSR has no effect.

Thanks,

	tglx