Peformance issues with r278325

[Stanislav Sedov]

> On Mar 18, 2016, at 11:49 AM, Ryan Stone <rysto32 at gmail.com> wrote:
> 
> On Fri, Mar 18, 2016 at 1:37 PM, John Baldwin <jhb at freebsd.org> wrote:
> I think I'll likely just convert it to use a direct
> TSC delay loop always in HEAD (assuming that verifies ok in testing as well).
> 
> Couldn't that work incorrectly on VM guests?  The tsc is not guaranteed to be monotonic in that environment.
> 

Another concern is SMP systems where the clock is not synchronized.  SMP TSC requires
a complicated setup procedure on the system boot which is not followed properly by all
vendors, and I have seen some recent systems with SMP TSC skew.

I'm afraid that using TSC in this code will make FreeBSD unusable on such (arguably buggy)
systems. 

--
Stanislav Sedov
ST4096-RIPE