Re: Performance overhead of get_cycles_sync

[Andi Kleen <andi@firstfloor.org>]

[headers rewritten because of gmane crosspost breakage]

> In the latest kernel (2.6.24-rc3) I noticed a drastic performance 
> decrease for KVM networking.

That should not have changed for quite some time.

Also it depends on the CPU of course.

> The reason is many vmexit (exit reason is cpuid instruction) caused by
> calls to gettimeofday that uses tsc sourceclock.
> read_tsc calls get_cycles_sync which might call cpuid in order to 
> serialize the cpu.
>
> Can you explain why the cpu needs to be serialized for every gettime call?

Otherwise RDTSC can be speculated around and happen outside the protection
of the seqlock and that can sometimes lead to non monotonic time reporting.

Anyways after a lot of discussions it turns out there are ways to archive
this without CPUID and there is a solution implemented for this in ff
tree which I will submit for .25. It's a little complicated though
and not a quick fix.

> Do we need to be that accurate? (It will also slightly improve physical 
> hosts).
> I believe you have a reason and the answer is yes. In that case can you 
> replace the serializing instruction
> with an instruction that does not trigger vmexit? Maybe use 'ltr' for 
> example?

ltr doesn't synchronize RDTSC.

-Andi