Re: [PATCH v2 0/3] x86/vdso: Add Hyper-V TSC page clocksource support

[Thomas Gleixner]

On Wed, 15 Feb 2017, Vitaly Kuznetsov wrote:
> Actually, we already have an implementation of TSC page update in KVM
> (see arch/x86/kvm/hyperv.c, kvm_hv_setup_tsc_page()) and the update does
> the following:
> 
> 0) stash seq into seq_prev
> 1) seq = 0 making all reads from the page invalid
> 2) smp_wmb()
> 3) update tsc_scale, tsc_offset
> 4) smp_wmb()
> 5) set seq = seq_prev + 1

I hope they handle the case where seq_prev overflows and becomes 0 :)
 
> As far as I understand this helps with situations you described above as
> guest will notice either invalid value of 0 or seq change. In case the
> implementation in real Hyper-V is the same we're safe with compile
> barriers only.

On x86 that's correct. smp_rmb() resolves to barrier(), but you certainly
need the smp_wmb() on the writer side.

Now looking at the above your reader side code is bogus:

+       while (1) {
+               sequence = tsc_pg->tsc_sequence;
+               if (!sequence)
+                       break;

Why would you break out of the loop when seq is 0? The 0 is just telling
you that there is an update in progress.

The Linux seqcount writer side is:

    seq++;
    smp_wmb();
    
    update...
    
    smp_wmb();
    seq++;

and it's defined that an odd sequence count, i.e. bit 0 set means update in
progress. Which is nice, because you don't have to treat 0 special on the
writer side and you don't need extra storage to stash seq away :)

So the reader side does:

    do {
    	  while (1) {
    	     s = READ_ONCE(seq);
	     if (!(s & 0x01))
	        break;
     	     cpu_relax();
         }
	 smp_rmb();

	 read data ...

	 smp_rmb();
   } while (s != seq)

So for that hyperv thing you want:

    do {
    	  while (1) {
    	     s = READ_ONCE(seq);
	     if (s)
	        break;
     	     cpu_relax();
         }
	 smp_rmb();

	 read data ...

	 smp_rmb();
   } while (s != seq)

Thanks,

	tglx