RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method

[Thomas Gleixner]

On Fri, 10 Feb 2017, Stephen Hemminger wrote:

> Since sequence count algorithm is done by hypervisor, better to not reuse seqcount.
> Still concerned that the code is racy.

That's a different question and can only be answered by the hypervisor
folks. Dunno, whether they have barrier requirements. The seqcount stuff
relies on:

do {
	seq = READ_ONCE(s->sequence);

	smp_rmb();

	sample_data();
  
	smp_rmb();
} while (s->sequence != seq);

which pairs with the writer side:

       s->sequence++;
       
       smp_wmb();

       update_data();

       smp_wmb();

       s->sequence++;

That's important if the stuff happens cross CPU. If the update happens on
the same CPU then this is a different story and as there are VMexits
involved they might provide the required ordering already. But I can't tell
as I have no idea how that host side thing is done.

Thanks,

	tglx