KVM: x86 shared msr infrastructure

The various syscall-related MSRs are fairly expensive to switch.  Currently
we switch them on every vcpu preemption, which is far too often:

- if we're switching to a kernel thread (idle task, threaded interrupt,
  kernel-mode virtio server (vhost-net), for example) and back, then
  there's no need to switch those MSRs since kernel threasd won't
  be exiting to userspace.

- if we're switching to another guest running an identical OS, most likely
  those MSRs will have the same value, so there's little point in reloading
  them.

- if we're running the same OS on the guest and host, the MSRs will have
  identical values and reloading is unnecessary.

This patch uses the new user return notifiers to implement last-minute
switching, and checks the msr values to avoid unnecessary reloading.

Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index b84e571f4175..4cd498332466 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -28,6 +28,7 @@ config KVM
 	select HAVE_KVM_IRQCHIP
 	select HAVE_KVM_EVENTFD
 	select KVM_APIC_ARCHITECTURE
+	select USER_RETURN_NOTIFIER
 	---help---
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