tree 2192140fe822f0805d3b766f3e277798e12445b0
parent 6c6e8360b34d59d2f687a1649e61173742dbc891
author Andrew Jones <drjones@redhat.com> 1493238744 +0200
committer Paolo Bonzini <pbonzini@redhat.com> 1493295377 +0200

KVM: add explicit barrier to kvm_vcpu_kick

kvm_vcpu_kick() must issue a general memory barrier prior to reading
vcpu->mode in order to ensure correctness of the mutual-exclusion
memory barrier pattern used with vcpu->requests.  While the cmpxchg
called from kvm_vcpu_kick():

 kvm_vcpu_kick
   kvm_arch_vcpu_should_kick
     kvm_vcpu_exiting_guest_mode
       cmpxchg

implies general memory barriers before and after the operation, that
implication is only valid when cmpxchg succeeds.  We need an explicit
barrier for when it fails, otherwise a VCPU thread on its entry path
that reads zero for vcpu->requests does not exclude the possibility
the requesting thread sees !IN_GUEST_MODE when it reads vcpu->mode.

kvm_make_all_cpus_request already had a barrier, so we remove it, as
now it would be redundant.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>