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

[Shaohua Li]

On Wed, Aug 05, 2015 at 10:44:24AM +0200, Ingo Molnar wrote:
> 
> * Shaohua Li <shli@xxxxxx> wrote:
> 
> > On Sun, Aug 02, 2015 at 09:41:08PM +0200, Thomas Gleixner wrote:
> > > 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 âWRMSR to APIC
> > >   registers in x2APIC modeâ 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.
> > 
> > What you quoted is for x2APIC, I didn't see similar description for
> > xAPIC.
> > 
> > Tested mfence here, it does work. But I'm not convinced it's the right thing. 
> > the xAPIC access is memory mapped IO, mfence is nothing related to it. [...]
> 
> Huh?
> 
> Can you cite the SDM that says that MFENCE will have no effect on instructions 
> that deal with accesses that are not to system RAM (i.e. are memory mapped IO)?

Hmm, I didn't mean mfence can't serialize the instructions. For a true
IO, a serialization can't guarantee device finishes the IO, we generally
read some safe IO registers to wait IO finish. I completely don't know
if this case fits here though.

Thanks,
Shaohua
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