x86/kvm: Support the vCPU preemption check

x86/kvm: Support the vCPU preemption check

Support the vcpu_is_preempted() functionality under KVM. This will
enhance lock performance on overcommitted hosts (more runnable vCPUs
than physical CPUs in the system) as doing busy waits for preempted
vCPUs will hurt system performance far worse than early yielding.

struct kvm_steal_time::preempted indicates that if one vCPU is running or
not after commit "x86, kvm/x86.c: support vCPU preempted check".

 unix benchmark result:
 host:  kernel 4.8.1, i5-4570, 4 cpus
 guest: kernel 4.8.1, 8 vcpus

         test-case                       after-patch       before-patch
 Execl Throughput                       |    18307.9 lps  |    11701.6 lps
 File Copy 1024 bufsize 2000 maxblocks  |  1352407.3 KBps |   790418.9 KBps
 File Copy 256 bufsize 500 maxblocks    |   367555.6 KBps |   222867.7 KBps
 File Copy 4096 bufsize 8000 maxblocks  |  3675649.7 KBps |  1780614.4 KBps
 Pipe Throughput                        | 11872208.7 lps  | 11855628.9 lps
 Pipe-based Context Switching           |  1495126.5 lps  |  1490533.9 lps
 Process Creation                       |    29881.2 lps  |    28572.8 lps
 Shell Scripts (1 concurrent)           |    23224.3 lpm  |    22607.4 lpm
 Shell Scripts (8 concurrent)           |     3531.4 lpm  |     3211.9 lpm
 System Call Overhead                   | 10385653.0 lps  | 10419979.0 lps

Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: David.Laight@ACULAB.COM
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: benh@kernel.crashing.org
Cc: boqun.feng@gmail.com
Cc: borntraeger@de.ibm.com
Cc: bsingharora@gmail.com
Cc: dave@stgolabs.net
Cc: jgross@suse.com
Cc: kernellwp@gmail.com
Cc: konrad.wilk@oracle.com
Cc: linuxppc-dev@lists.ozlabs.org
Cc: mpe@ellerman.id.au
Cc: paulmck@linux.vnet.ibm.com
Cc: paulus@samba.org
Cc: rkrcmar@redhat.com
Cc: virtualization@lists.linux-foundation.org
Cc: will.deacon@arm.com
Cc: xen-devel-request@lists.xenproject.org
Cc: xen-devel@lists.xenproject.org
Link: http://lkml.kernel.org/r/1478077718-37424-10-git-send-email-xinhui.pan@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>