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Subject: Re: [RESEND PATCH v4] x86/hpet: Reduce HPET counter read contention
To: Andy Lutomirski <luto@amacapital.net>, Waiman Long <waiman.long@hpe.com>
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From: Dave Hansen <dave.hansen@intel.com>
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Date: Fri, 12 Aug 2016 14:16:09 -0700
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On 08/12/2016 01:18 PM, Andy Lutomirski wrote:
> I don't think this is right.  If the HPET ever returns the same value
> twice in a row (unlikely because it's generally too slow to read, but
> it's plausible that someone will make a fast HPET some day), then this
> could deadlock.

True...

I guess that means we've got to do some kind of sequence counter
preferably in the same cacheline as the HPET value itself, or _something
that we guarantee to change on each write to the cached value.

> Also, does this code need to be NMI-safe?  This implementation is
> deadlocky if it's called from an NMI.

Urg.  Can't we just do

	if (in_nmi())
		return read_real_hpet();

?

> The original code was wait-free, right?  That was a nice property, too.

You mean no spins?  I don't think this one really spins ever either.