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

[Ingo Molnar <mingo@kernel.org>]

* Shaohua Li <shli@fb.com> 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 “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.
> 
> 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)?

Thanks,

	Ingo