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KVM: add explicit barrier to kvm_vcpu_kick

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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>
This commit is contained in:
Andrew Jones
2017-04-26 22:32:24 +02:00
committed by Paolo Bonzini
parent 6c6e8360b3
commit cde9af6e79
3 changed files with 7 additions and 4 deletions
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2
arch/x86/kvm/x86.c
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@@ -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

6
include/linux/kvm_host.h
View File

@@ -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);
}

3
virt/kvm/kvm_main.c
View File

@@ -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);