x86, vdso, pvclock: Simplify and speed up the vdso pvclock reader

x86, vdso, pvclock: Simplify and speed up the vdso pvclock reader

The pvclock vdso code was too abstracted to understand easily
and excessively paranoid.  Simplify it for a huge speedup.

This opens the door for additional simplifications, as the vdso
no longer accesses the pvti for any vcpu other than vcpu 0.

Before, vclock_gettime using kvm-clock took about 45ns on my
machine. With this change, it takes 29ns, which is almost as
fast as the pure TSC implementation.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/6b51dcc41f1b101f963945c5ec7093d72bdac429.1449702533.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>