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Commit cde9af6e authored by Andrew Jones's avatar Andrew Jones Committed by Paolo Bonzini
Browse files

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: default avatarAndrew Jones <drjones@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 6c6e8360

arch/x86/kvm/x86.c

+1 −1
Original line number Diff line number Diff line
@@ -6853,7 +6853,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) 


	/*

	 * 1) We should set ->mode before checking ->requests.  Please see

	 * the comment in kvm_make_all_cpus_request.

	 * the comment in kvm_vcpu_exiting_guest_mode().

	 *

	 * 2) For APICv, we should set ->mode before checking PIR.ON.  This

	 * pairs with the memory barrier implicit in pi_test_and_set_on

include/linux/kvm_host.h

+6 −0
Original line number Diff line number Diff line
@@ -270,6 +270,12 @@ struct kvm_vcpu { 


static inline int kvm_vcpu_exiting_guest_mode(struct kvm_vcpu *vcpu)

{

	/*

	 * The memory barrier ensures a previous write to vcpu->requests cannot

	 * be reordered with the read of vcpu->mode.  It pairs with the general

	 * memory barrier following the write of vcpu->mode in VCPU RUN.

	 */

	smp_mb__before_atomic();

	return cmpxchg(&vcpu->mode, IN_GUEST_MODE, EXITING_GUEST_MODE);

}

virt/kvm/kvm_main.c

+0 −3
Original line number Diff line number Diff line
@@ -183,9 +183,6 @@ bool kvm_make_all_cpus_request(struct kvm *kvm, unsigned int req) 
		kvm_make_request(req, vcpu);

		cpu = vcpu->cpu;



		/* Set ->requests bit before we read ->mode. */

		smp_mb__after_atomic();



		if (!(req & KVM_REQUEST_NO_WAKEUP))

			kvm_vcpu_wake_up(vcpu);